Siege Warfare - Part 1

For most of the period known as the Middle Ages and onwards into the 20th Century, sieges have been a recognized and regular form of warfare. Whether the defenders were in a traditional castle or walled town, or simply holding a piece of territory with strong defences, the process of attacking is known as a 'siege'. I will use the term 'fortress' throughout for the sake of keeping it simple.

In this piece, I will deal with sieges typical of the period 1600-1850, but the principles can be extended over a wider period.

Sieges were usually undertaken in the course of a campaign in another country or during a civil war, when the attacking army needed to make progress but was loth to leave a defended fortress in its rear. There was always the danger of defenders in these places sallying forth to attack the army's lines of communication. The answer was to 'lay siege' to the fortress and capture it.

This was a specialized form of battle. The procedure followed a fairly set pattern, predictable to such an extent that, by the time of the Peninsular War, it was possible to say that a given fortress could only hold out for about 40 days. Attackers needed to get the attack over before any other force could come to the relief of the defenders, and the commander of the attacking force had to take into account the possibility of coming under attack himself.

The first move would be to throw a ring around the fortress, preventing anyone from getting out as well as holding off any relief attempts. Next came the process of capturing any out-works - key places in the outer defences that were designed to make it difficut to attack the main fortress. To breach the defences themselves - usually a system of high walls and ditches, designed by experts in fortification - it was a question of getting guns close enough to the chosen point of attack to be effective. They had to make a hole wide enough for the infantry to pour through and attack the defences directly.

The hitting power of a cannonball falls as range increases, simply because it loses speed due mainly to air resistance. Thus the closer the guns could get to the walls, the more damage they could do and the faster they would force a breach in the walls. But the fortress was usually well equipped with guns of its own and these were often bigger guns than those of the besieging force because they were not having to be moved about on the campaign. The attackers therefore had to work their way forward carefully to avoid the guns being destroyed by 'counter-battery' fire.

They did this by digging long trenches, or saps, that aligned diagonally forward at an angle that prevented the defenders from looking along them and seeing what was going on. Men and materials were moved forward to positions that were often developed during the hours of darkness, where guns could be emplaced on firm platforms with protection in the form of gabions. A gabion was a large wickerwork basket filled with earth or stones - these were often made using local labour. These would be stacked up on the front of the firing platform with gaps through which the guns could fire. A gabion could in fact be anything that would absorb the power of incoming shot and there is an account of one siege where the attackers found a warehouse full of baled sheeps' wool, and used these to good effect.

                                                               Woolsacks as gabions

There would often be two or three new firing positions as the batteries moved closer until they were as little as 200 yards from the walls, from which distance they could cause enormous damage, although it could also mean they were in greater danger themselves from the defenders' firepower. In addition, there would be attacks at different points on the walls to keep the defenders guessing as to the intended main attack.

Neutralizing the defenders' guns was important for obvious reasons and this was the task of field batteries, often equipped with howitzers and mortars at longer ranges. Their role was to bring fire down on the fortress's guns and on defenders who might be trying to reinforce their positions behind the expected point of breaching.

It seems remarkable, but the breaching batteries could quite accurately 'cut' a wall, literally carving out a section with vertical 'cuts' of the width they required for a breach and then cutting a line across the bottom of the section so that the whole wall slid to the ground - more often into the ditch that usually surrounded the walls. This not only got rid of the wall, but also provided a means of filling the ditch and making it a bit easier to cross.

Gun construction in wrought iron and bronze

It seems probable that the first material used in making guns was a copper alloy similar to brass or bronze. The earliest guns in the Royal Artillery Museum are Chinese and, although one of them is an early cast iron, the other is a copper alloy piece of a type that seems to have been made for many years - it even has a production number.



These copper alloys are easier to work than iron, which requires a higher melting temperature in order to make cast iron or a skilled blacksmith for wrought iron. Casting in copper is relatively straightforward and needs little imagination, but we will come to that presently. Let's start with wrought iron.

In Europe in the days before iron could be melted, it was worked by heating it to a glowing red and hammering it on an anvil. That is how horseshoes are made and the procedure for making guns followed the same path. Manufacture began with a sheet of iron that had been hammered out from a block and then wrapped around a mandrel - a solid cylindrical block of the required calibre for the gun itself. This formed the inner 'tube' or lining of the gun. Long rods of iron made like staves were then laid out along the length of the gun and held in place by a series of iron hoops. These were slipped into place while still hot, so that as they cooled and shrank, they tightened the rods in place. This built-up construction produced a tube open at both ends. Onto one end was fitted a 'breech block', often in the shape of a beer mug, complete with a handle and a vent hole. Holding that in place was usually a suitably shaped mounting carved out of a solid block of wood, while the rest of the gun was strapped tightly to that same mounting.



This construction, using long staves and hoops, was very similar in concept to the cooper's method of making wooden casks. Another name for a cask is a barrel - hence the name 'barrel' for the long tube of a gun!

This method of making guns was extremely laborious and the guns themselves tended to leak gas pressure, especially around the breech area, and were frequently not strong enough, blowing apart and injuring the gunners who served them.

Casting guns in copper was much preferred, though copper itself was expensive and had to be imported into England from the Continent. The skills also had to be imported and it was Henry VIII who had the wealth and determination to do just that, persuading a Venetian gunfounding family, the Arcana, to come to England and start making guns for the King. The procedure was relatively simple in concept, if rather more complex in practice.

It began with the making of a maquette - an exact replica of the external dimensions of the gun, complete with decoration (see picture), made in clay. This was actually made on a former - a length of wood wrapped in coils of thin rope, then covered in clay. When the maquette had dried, it was lightly greased and then the mould was built up around it in clay. When this, too had dried, the maquette was broken out of the mould - this was where the rope coils came in, since these could easily be pulled out, together with the former, leaving just the relatively thin clay to be broken away.



The mould was then thoroughly dried before being buried in a casting pit, muzzle end upwards, for the molten metal to be poured into it. In the years before the late 18th Century, a metal rod would be placed inside the mould and secured so that the molten metal would run around it, forming the inner shape of the barrel. However, it was found that casting the piece as a solid and drilling out the bore of the gun produced a stronger barrel.

Artillery Ammunition - Part 1: Smoothbore period

The real weapon of the artillery is the bit that does the damage, i.e. the ammunition, not the gun or mortar though, of course, the rocket is both the projector and the ammunition.

The earliest ammunition types were the war arrow, a hail of large stones and the stone cannonball. It would be difficult to say which came first, though the war arrow is the munition depicted in the earliest known image of a gun.



There seems to be no record of them being used in war, but they remained in the inventory of the Tower of London for some 200 years. They would have had to have the warhead and the fins of the same diameter as the bore of the gun and be loaded with a tampion or wad between the tail and the propellant to provide the seal that allowed the propellant to develop its pressure. Their disadvantage was that they were only effective against men or cavalry in the open and did not have the 'hitting power' of a cannonball in siege warfare.

The hail of stones was a weapon of the 'perrier' mortar and was useful in sieges for attacking people in the open, thinking themselves protected by city or fortress walls. The stone cannonball was really only a replica of the projectiles hurled by the catapult and trebuchet, but in guns it had to be cut more accurately if it was to do the job of sealing the propellant gases long enough for them to build up pressure, and of course, not to damage the gun.

When guns began to be cast in bronze, there were occasional examples of bronze cannonballs, but the more usual early shot from these guns was the stone ball with a covering of lead. The lead gave it the weight to make it carry further and yet avoided the problems in making it entirely of lead and too heavy. Too much weight produces greater resistance in the bore and could result in bursting the gun.

Once the technique required for melting iron had been discovered, guns were cast in this material, producing stronger guns that could cope with heavier ammunition and larger propelling charges. Cannonballs began to be made of iron, usually by the same people who were casting iron guns because they had both the material and the requisite skills.

Throughout the early period and right up to the end of the smoothbore period in the middle of the 19th Century, incendiary projectiles were much in evidence. They were used in siege warfare to set fire to the wooden constructions and thatched roofs found inside cities and castles. Based on a simple internal framework, the materials used to construct the fireball varied over the years, but they must have been a frightening sight for the defenders as they flew across the sky, flames burning brightly and trailing smoke.



The invention of the hollow shell led to the development of other types of ammunition. Filled with gunpowder and detonated by a long fuze, shells were very effective in causing both damage and casualties. The blast effect, with its noise and the flying fragments of the burst shell, was a new terror both in siege warfare and on the battlefield. When Henry Shrapnel packed a shell with musket balls and burst the shell above the heads of the enemy, the rain of lead balls was, in effect, a longer range version of 'canister', the dreaded hail of lead fired at close range against infantry and cavalry.

And last, the extraordinary invention of light-producing flares to illuminate the battlefield or the breach in a siege wall, including during the 19th Century a flare packed with a parachute that would take longer to fall, making it more useful.

What's the connection between artillery and spiders?

Strange as it may seem, there is one and it's even written up in an official 20th Century pamphlet on artillery instruments.

Put very simply, spiders spin a tough silken thread that makes an excellent 'graticule' in an optical sight, such as those used in artillery instruments. Graticules are the fine lines in a sighting system that are used to lay an instrument accurately on a given point. It was found that the web of a common spider in the grounds of the Royal Arsenal could be used, and these were sought out so that long strands could be selected and wound on a frame. A short length would then be sandwiched between thin sheets of optical glass and inserted at the correct point in the lens system for the sight so that it would remain in focus when the sight was being aimed, whether at a close or a distant object.

I doubt whether it's a system still in use, though graticules remain an essential element of many types of sight and of military binoculars and telescopes. Spiders are probably considered old-fashioned - a bit like gravity, but that's another story!

Artillery Instruments

One of the most basic instruments used in artillery is that for determining the elevation angle of a gun. It was known from the earliest experiments with guns that a given angle of elevation together with a fixed amount of propelling charge would achieve (theoretically!) the same trajectory every time the gun was fired. Variations in the quality of the propellant, the weight of the projectile, the condition of the bore, the weather conditions etc. all affected this trajectory, but for their purposes, gunners were able largely to rely on this angle of elevation to achieve a known range.

It was therefore important to be able to set this angle on the gun. Their solution was the gunner's quadrant. This simple device had an arm that went into the barrel, a quarter circle (quadrant) measuring instrument and a plumb bob. As the gun elevated, so the plumb bob's string moved on the quadrant, allowing the gunner to read the angle.

Over time, of course, this device became gradually more sophisticated, acquiring a levelling bubble to replace the plumb bob and achieving very high accuracy by the means of precision engineering. However, in essence, that is all that a gunner's quadrant is - a means of measuring the angle of elevation of a gun's barrel.

Concentrating Fire

After the major changes that took place in artillery at the end of the 19th Century - improvements in ammunition effectiveness, in range and accuracy, in survey, in communications and in the relatively new art of indirect fire - gunners began to use the new capabilities to concentrate fire.

In essence, this meant bringing down artillery fire on a target from more than a single battery. Now that they had the communications to pass target details to other batteries, it became possible to engage a target with every battery in range. The more that ranges improved, the more batteries it became possible to concentrate on a given target.

This in turn drove the pressure for further improvements in communications, with telephone lines and radio frequencies dedicated to artillery use, and procedures for command and control that established the priorities to be followed in engaging targets.

Survey methods were developed that put all batteries on the same 'grid' - i.e. knowing exactly where each one was in relation to all the others - and ensuring that they had a common 'orientation', so that when their sights were pointing at 'grid north', for example, all guns were parallel. When this was achieved, it became possible for one battery to be adjusted onto a target and any adjustments to the 'map bearing and range' by the 'ranging battery' could be applied to the map bearing and range of any other battery.This meant in effect that any other battery within range could engage the target without further adjustment. This not only saved time and ammunition, but provided no warning to the target about how many guns were about to engage it.

I have simplified the process to its essentials: it was in practice rather more complicated, but it was a major factor in World War 1, the first 'artillery war'. The stalemate of the trenches made attacking across the no man's land between them a deadly lottery. Artillery was needed to suppress enemy firepower whilst infantry crossed in the open to come to grips with their opposition. Complex fire plans were evolved, tasking batteries to engage targets in preparation for an attack, sometimes for days at a time, pounding at defences in an attempt to weaken strongpoints, cut barbed wire and crush morale. This preliminary bombardment would be followed by firing at specific targets at set times to coincide with infantry movement, aiming to keep the enemy's infantry hiding under cover while the attack developed across open ground. Batteries would then 'lift' to engage targets in rear of the front lines, aiming to break up counter-attacks and to suppress enemy batteries.

One of the new methods of fire involved 'creeping barrages', with the supporting fire falling in lines just in front of the advancing troops and lifting line by line as little as 50 yards ahead of the infantry. These barrages were immensely complicated to work out and to achieve, but they were nonetheless a useful aid in keeping the infantry moving in the right direction in the fog of war and in preventing the enemy machine-gunners from mowing down the unprotected infantry.

By the time of World War 2, the Allied firepower was so great that they could afford to hit anything that moved with devastating effect. This led to rapid advances in the closing year of the War as the German armies were forced back across north west Europe. The lesson was rammed home again in the first Gulf War: concentrated artillery firepower remains the Queen of the Battlefield.

Factors affecting Indirect Fire

Engaging targets at indirect fire - i.e. targets that can not be seen from the gun position - is quite a complex business. In an ideal situation, it should be possible simply to determine where a target is in relation to a gun in terms of a compass bearing and a distance, to set these on an appropriate sighting system on the gun and to hit the target with the first round fired. Unfortunately for gunners, situations are seldom ideal!

Let us look at some of the factors that affect indirect fire.

The gunner has first to know exactly where on the map his gun is sited, and to ensure that when his gun is pointed on a given compass bearing, it is truly pointing on that bearing. It is surprisingly easy to be wrong on both accounts, which can obviously result in missing the target. It is of course important for the target's position to be accurately located on that same map grid, since the bearing and range to it are determined by comparing the two locations. In effect, a line drawn on the map between the gun and target provides the bearing (the angle between that line and grid north), while the length of the line is the scale of the distance, or range.

But even when those pieces of information are known, the gun has to be able to shoot 'to the map' - i.e. perform to a set of standards that are laid down as 'normal conditions'. When a gun is calibrated, the gunner determines to what extent its performance matches the standard set for that gun. There are many factors that can make it differ in practice and these have to be taken into account in setting target information on the sights. The main non-standard condition on the gun itself is any difference in muzzle velocity for the charge being fired. Gun barrels get worn by the corrosive gases and the passage of shells, and as they wear they become less able to achieve the standard muzzle velocity. A lower than standard velocity means the shell will not go as far as it should when the range is set at a given distance (angle of elevation), so that non-standard muzzle velocity has to be taken into account by increasing the range set.

Another important factor at the gun is the ambient temperature of the propelling charge. Propellant burns faster when the ambient temperature is above standard, leading to higher muzzle velocity, so again, compensation is needed. Projectiles can vary in weight, and although the weight differences from round to round are not large, they nevertheless have an effect on the range achieved and need compensation.

It is obvious that the shell is affected in flight by atmospheric conditions - wind speed and direction are obvious examples, but perhaps it is not as well known that these can vary at the different levels along the trajectory and require compensation in the settings on the gun before firing. Barometric pressure affects air density and that determines what air resistance is offered to the shell in flight. On a long trajectory, even the rotation of the Earth can become a significant factor and need compensating adjustments.

Some factors, such as 'drift', are built in to all situations. Drift is the tendency of a spinning projectile to build up pressure on one side and hence drift away from that pressure. That effect is well-known and range tables automatically include it.

These are the most common factors. There are others, but this will be sufficient to make the point that engaging targets at indirect fire is a complex business!

Firepower - A Quantum Leap

In having the most dominant firepower, artillery has long been the "Queen of the Battlefield", but its reign began to falter towards the end of the 19th Century, when infantry weapons made enormous steps forward in their rate of fire, range and accuracy. Up to that point, artillery had by far the longer range and, in addition to cannonballs and explosive shells, it could deliver punishing salvos of cased shot (canister) at infantry and cavalry targets long before they could retaliate with their own weapons. It remained vulnerable to enemy artillery, of course, but that was true of both sides, and artillery tended to devote most of its effort towards reducing the enemy's infantry and cavalry forces.

This new infantry firepower had the effect of driving field artillery back from the front line: it was too valuable a resource to leave it in positions where gunners and horses could be picked off by skilled riflemen. After moving out of range of the infantry weapons, it still had the dominating firepower, but the field guns of that period had to be able to see their targets. Frequently these targets were out of sight and guns had to be layed for indirect fire: this brought about a whole new era of change.

But the gunner first needed to know where the target was, so someone had to be far enough forward to be able to see it and to pass that information to the gun position. In addition, he had to act as an observer of the fall of shot when the target was engaged, so that corrections to the aim could be given. Some means had to be found of pointing the guns at their 'invisible' target. This was resolved by using maps with a grid system that allowed guns and their targets to be plotted accurately. This enabled bearing (direction) and range to be measured, so that this information could be used to point the guns and to set the correct elevation.

A forward observer was not able to communicate with the gun position unaided, so signalling systems had to be developed to pass orders and corrections between them. Signal flags (semaphore), heliographs and telephones were stages in the progress towards radio communications - all unnecessary, of course, in the days when guns were in the front line.

New instruments were needed on the guns, too. A means of sighting the gun, using a simple form of theodolite and compass, meant that guns could be reliably pointed at any required bearing. The correct range was set using data from range tables coupled with a form of gunner's quadrant. Sighting systems that were initially rudimentary soon became quite advanced, thanks to the technical skills developed in other branches of artillery, such as coast guns.

The combination of these changes freed field artillery from the space constraints of the front line, where there was insufficient room for more than a few guns per battalion of infantry. Suddenly there was an enormous amount of room, since guns soon had enough range not only to clear their own front line and hit the enemy, but also to hit targets well behind the enemy's front lines.

By the time of the Great War in 1914, most of the problems of indirect fire had been ironed out and artillery was being massed to provide devastating firepower, vastly more than had ever been envisaged in the days of direct fire. It was truly a quantum leap and maintained the old tradition of "Queen of the Battlefield".

Muzzle loading versus Breech loading

The idea of loading an artillery piece at the breech end is not a 19th Century development. It is the logical place to load and it was practised in 15th Century wrought iron guns (see separate article) for sound reasons. On a ship, for example, a long cannon that has to be loaded at the muzzle end has to be pulled right back in order to get at the muzzle. It would not only be difficult to put a heavy cannonball into the muzzle if it was sticking out over the side of a ship, especially in heavy seas: it would be virtually impossible to ram it all the way down the bore to the breech end. It was much easier to leave the gun in place and simply open up the back end to load the ammunition.

Unfortunately, it was hard to make an opening that could be tightly sealed before firing the gun. Gas under high pressure will escape out of every tiny crevice and since it was both extremely hot and highly corrosive, it wore away metal and made the holes larger. It also made the gun very dangerous to operate! It soon became apparent that it would be better to stay with the original idea of a tube with only one opening (at the muzzle) and to load the ammunition from that end. This required only one small hole at the breech end in order to let in the flame that would light the propellant and fire the gun. This 'vent' was subject to the same problems of corrosive wear, of course, and needed to be repaired from time to time.



Muzzle loading had its dangers, too. After firing there was almost always some hot residue in the chamber - i.e. the breech end of the barrel, where the ammunition is placed. There was a risk that the next charge of propellant pushed down to the chamber would ignite prematurely and injure the gunner who was loading it. There were two basic precautions taken against this: one was to swab the bore with a wet sponge; the second was to cover the vent hole with a large thumb while the propellant was being loaded. This prevented a flow of air being created by the ramming of the propellant and the fanning of any residue of spark into flame.

Breech loading was resurrected in British artillery in the 19th Century when multi-groove rifling made it very difficult to load ammunition from the muzzle end. Various designs were tried, but it was the design by William Armstrong that was adopted in 1858. This had a 'breech block' that could be lifted out to allow ammunition to be loaded into the chamber of the gun. When replaced, a screw mechanism pushed it tight against the rear face of the barrel and sealed it - a process known as 'obturation'. This block contained the vent for firing.

Later designs also used a breech block, but they were sliding blocks, sometimes operated horizontally, but mostly vertically. These incorporate the firing mechanism, usually a percussion mechanism like that used in a small arm. The striker in the mechanism either fires a small cartridge that sends hot gases into the propellant, or activates a priming tube in the base of a cartridge that holds the gun's propellant. (Note that, depending on the design of the gun, propellant can be loaded either in bags or contained in a large 'brass' cartridge case.)

Despite Armstrong's achievements, the higher cost of rifled breech loaders (RBL) caused the authorities to revert to muzzle loaders (RML) for some 20 years, albeit with a rudimentary rifling system based on fewer grooves and projectiles with projecting studs. (See a later article on aspects of rifling.) Eventually, designers reverted to RBL and that is how most guns in service use are designed today.

Smoothbore to Rifling

For the better part of 600 years, artillery weapons were smoothbores: in other words, the inside lining of the barrels was smooth, with no means of influencing the spin of the projectiles. Nevertheless, projectiles almost always spun because, as they rattled up the bore, bouncing from one contact with the bore to another on the way, the final contact before emerging into free air imparted a slight drag at that point and thus made the projectile spin. It was pure chance which direction the ball spun, and since a spinning ball (like a sliced golf or tennis ball) will veer off line, this built in an element of unpredictable behaviour. It was less apparent in a high speed shot from a cannon than in a lower speed shot from a howitzer, but it was still an unwanted effect.

Rifling was a system of grooves cut in a spiral along the lining of the barrel. The projectile's surface was forced into following the spiralling grooves, emerging with a constant, defined spin. This had two advantages: first it provided a predictable effect that could be compensated for; second it provided stability along the trajectory due to the gyroscopic effect of the spin. This became particularly important for artillery when it moved from firing spherical shot and shells to cylindrical munitions.



Unlike a spherical shape which presents the same aspect to the air all along its trajectory, unspun cylindrical projectiles from a smoothbore are quickly forced by air pressure to present their largest surface to any resistance and hence tumble in flight, severely affecting both range and accuracy. To avoid this they need to be gyroscopically stabilized by spinning, or else fitted with fins. Tail fins, like the feathers on arrows, can stabilize a projectile in flight. However, while there are occasions when fitting fins is a useful option, it is not regarded as a good solution for the bulk of "tube" artillery because the fins add to design problems inside the barrel.

Attempts were made to introduce a form of rifling quite early (as far back as the middle of the 15th Century), and by the 17th Century it was shown to be effective in sporting weapons. However, it was considered to be too expensive for military use until the end of the 18th Century, and even then it was only used for specialized troops like sharpshooters. It was still not taken up in the artillery world because it was not considered necessary, but all that changed in the 19th Century.

One of the major factors driving the change was the recognition that the weight of fire from a given calibre of gun could be significantly increased by moving from a spherical munition to a cylindrical shape: it meant that the projectile could contain a much larger amount of explosive. To get that amount of explosive inside a spherical shape would mean a larger diameter ball and in turn a larger diameter barrel and a heavier gun.

But rifling had a significant disadvantage: it was much harder to load a gun by pushing the projectile down the barrel from the muzzle end, as had been the practice with smoothbores. That meant opening the back (breech) end of the barrel and then having to close it with sufficient strength in the design to cope with the very high pressure in the chamber on firing. Solving this problem is a big subject that I will tackle in another article.

Cannons, Mortars & Howitzers - "horses for courses!"

While the term 'gun' is a catch-all, there are other names in the artillery world that have a more specific meaning. Cannons, mortars and howitzers are specific types of artillery and their roles are markedly different from one another. It may be useful to explain these roles as an aid to understanding how artillery developed.

This article will briefly set out the roles in the period of smoothbore artillery - i.e. before the middle of the 19th Century, when there was a major upheaval in the development of artillery. (Smoothbore versus rifled artillery and the meaning of those two terms will appear in another article in due course.)

Cannons and mortars were the earliest of the artillery weapons. The essential difference between the two lay in their trajectories. The cannon fired a solid cannonball with a large charge of gunpowder to propel it out of the barrel at a high speed: the cannonball gradually slowed down as it travelled along its very 'flat' trajectory. This slowing down produces the curved shape of a trajectory as gravity gradually takes a greater effect and pulls the projectile downwards.

The cannon's role on the battlefield was like that of the bowler in a bowling alley, knocking down skittles, only these were soldiers or horses. In siege warfare, the cannon's task was to breach the walls of a fortress so that the infantry could get inside. These walls tended to be thick and high, and needed heavy shot fired at a high velocity. Clearly, then, a cannon was a 'direct fire' weapon - it engaged targets it could 'see', just like a tank gun does today.



The mortar, on the other hand, was almost exactly the opposite. It lobbed shells in a trajectory that was like that of a tennis player's lob over his opponent's head - going steeply up and descending equally steeply. A mortar's maximum range (distance fired) tended to be much less than that of a cannon, but it could project its shells over intervening obstacles like fortress walls, small hills or woods. Mortar ammunition consequently developed along different lines. A mortar could deliver heavier projectiles and in the early days, they often fired a charge of large stones, showering these down on the those inside the fortress. Another much-used projectile was an incendiary that would cause fires in the timber constructions inside the fortress. Later it became easier to cast hollow shells and these usually had a filling of gunpowder to provide an explosion at the target end.

Inevitably, someone had the idea that it would be useful to combine the roles of a mortar and a cannon in a single weapon that could both lob exploding shells and project them to greater ranges. This new weapon was called a 'howitzer' and, of course, was a compromise: it was not possible to achieve either the greater weight of shot of the mortar or the range and hitting power of the cannon. However, it could do a bit of both roles and was regarded as a useful weapon. In the Royal Artillery at the time of the Napoleonic Wars, howitzers were often incorporated in a battery of cannons so that both capabilities were available on the manoeuvre battlefield.

Propellants and Explosives

There are two basic types of explosive: low explosives and high explosives. Most explosives detonate rather than burn. A detonation is a very rapid chemical reaction using oxygen that is contained in the material rather than in the air. In a detonation, the chemical reaction releases gases that rapidly expand and give off energy as they become hot, creating a pressure wave, or 'blast'.

However, low explosives tend to burn, giving off great heat and intense light, rather than detonate. This is known as 'deflagration'. Because they burn at a slower rate, this creates less pressure than high explosives and they are often used as propellants in guns and rockets: in other words, they provide the 'pushing' power that drives a rocket or sends the shell out of a gun.

From the time of its discovery until the mid-19th Century, black powder - or gun powder as it was usually known - was the most common explosive used throughout the world, and was used both as a propellant and (by enclosing it tightly in a confined space) to produce a detonation. However, it produced a lot of smoke, became ineffective when damp (hence the expression "Keep your powder dry!") and was dangerous to use - all factors that counted against it on the battlefield. Today black powder is still used for fireworks, special effects and other specialized work, but it has been replaced in warfare by more effective smokeless propellants and higher powered, yet safer, explosives.

It's worth spending a moment on how a propellant works in a gun because it was usually propellant improvements over the years that drove changes in gun design.

Gunpowder is a mixture of potassium nitrate (also known as saltpetre), charcoal and sulphur in a ratio of 15:3:2. The mixture varied over the years, but this became the generally accepted best mixture for guns. When a charge of gunpowder was placed in the chamber at the rear of a cannon and a cannonball loaded in front of it, the powder was in effect in a closed container. There was a vent hole immediately above the charge through which a flame was introduced to 'fire' the gun, and there was a small amount of space around the cannonball ("windage"), but there was not enough of these spaces to allow the propellant gases to escape, once the gunpowder was alight. The rapid burning and the release of huge quantities of gas raised the pressure in the chamber of the gun, and as the pressure increased, so did the rate of burning. Something had to give or there would be an explosion!

What gave, of course, was the cannonball. It was pushed at high speed along the barrel, accelerating all the way until it was ejected and sent off on its journey to the target. If the gun had a long barrel - like most cannons - it was accelerated for longer and therefore emerged at a higher speed than it would in a mortar, which all had much shorter barrels than cannons.

Much development went into making gunpowder burn faster, which increased the muzzle velocity, but in turn, this meant that the pressure at the chamber end of the gun became greater and required extra reinforcement. If you look at an early cannon, it is relatively slender compared with later guns, where the chamber end of the gun tends to be very much thicker than the muzzle end, due to the need for stronger, thicker walls to contain the pressure.

When is a 'tank' not a tank?

Quick answer - when it's a self-propelled gun!

It's quite common to hear people, especially in the media, referring to any tracked vehicle with a gun as a 'tank'. They are demonstrating their ignorance! There are so many points of difference that it's perhaps worth taking a moment to look at them.

First, the roles of a tank and a self-propelled gun are completely different. A tank is designed to operate in contact with the enemy and needs to have the mobility, the protection and the firepower for this role. A self-propelled gun is an artillery weapon and has to be capable of engaging the enemy across a wide sector of the front and in depth, working together with other artillery pieces to produce a great weight of fire on the chosen target.

A tank's gun is designed to knock out other tanks and fires high velocity projectiles that can penetrate very thick armour plate. It has other types of ammunition as well, but it is this high velocity role that drives the design of a weapon that has a very flat trajectory and superb sighting systems. Its armour is especially designed for protection against most forms of direct fire and that armour is concentrated on the front of the tank, facing the enemy. As a result, it is normal for tanks to have their engines in the rear, protected by all the frontal armour and out of harm's way. The thick armour means that tanks tend to be very heavy and the amount of space inside the turret very limited, in turn limiting the amount of ammunition that can be carried. However, tanks do not expect to have to fire large amounts of ammunition and their guns wear quite quickly - in this respect they are very different from artillery weapons.

A self-propelled (SP) gun does not need heavy frontal armour because it is not in direct contact with the enemy, but sited some 3 - 6 Km behind the front lines where its lighter armour provides adequate protection from shell fragments and small arms fire. Its artillery role means that it is required to be able to fire a lot of ammunition, so its carrying capacity is much greater than that of a tank. It may well fire in excess of 300 rounds during a single day and feeding ammunition into the turret is a major factor in its operation. It is a great help to be able to open the rear of the chassis to allow easy access, but that cannot be achieved with a rear-mounted engine, so that's usually in the front of the chassis, unlike the tank.

Both types of vehicle require good mobility, but in a tank it can be particularly important for its effectiveness and, sometimes, for its survival. Tanks tend therefore to have very powerful engines, not only for speed but also to cope with their weight in cross-country conditions.

The list could go on, but perhaps I could end with a fundamental difference - their sighting systems are completely different. Where tank sights are designed to deal with targets at direct fire, SP gun sights are intended to deal with targets at long range and out of sight.

Trajectories and Hoses!

When or why would you want to shape a trajectory?

Well, you almost certainly know how to do that already! If you have ever lobbed a ball over a wall to someone, you did it because you couldn't throw it directly - the wall was in the way. In lobbing it, you chose a trajectory that would clear the wall.

If there's nothing in the way, a soldier with a rifle simply aims directly at the target. The rifle can 'see' it and the bullet will follow a very 'flat' trajectory to the target. The same applies to a tank gunner: the gun can 'see' the target directly and the tank shell also follows a very flat trajectory to the target.

In the world of artillery, you frequently have to select a trajectory that will deliver a shell to a target when there's a hill in the way - in effect, you have to lob it over the hill. You may also have to deliver a shell to a target that is tucked away in a gully, where a normal trajectory is unable to reach it, so you need a high trajectory with a steep angle of descent. An ability to shape the trajectory to fit the conditions becomes an essential tool.

Have you ever handled a garden hose? If so, you will know how to shape trajectories using the same basic tools that an artilleryman uses - angle of elevation and pressure. You know that as you point the hose upwards, the water goes further down the garden - until you reach an angle of 45 degrees! After that, if you go on tilting the hose upwards, the watering point will start coming back towards you. If you point it at 90 degrees, which is directly upwards, you start to get wet!

Well, exactly the same happens with an artillery weapon. The range (distance) goes on increasing as you elevate the barrel of the gun until you reach that 45 degree mark. This part of the gun's performance is known as "low angle" firing. Above 45 degrees, the range starts to decrease just as it does with a hose. This is known as "high angle" firing.

But using a hose, you can also change the distance the water goes, even if you maintain the same angle of elevation. You simply turn the tap and change the water pressure. The higher the pressure, the further the water goes. In a gun, you change the pressure by adding or subtracting to the propelling charge. The larger the charge, the bigger the kick given to the shell and the faster it goes; so here, too, you can change the range without changing the angle of elevation.

With these two basic tools - angle of elevation and variation of charge - you can shape the trajectory to fit almost any situation. But note that I said "almost". There are often situations where a target cannot be reached from a given firing position and then you have to move the gun.

Welcome

With a long background of military service and a particular interest in military history, I became involved with the setting up of the Royal Artillery museum now sited in the Royal Arsenal, Woolwich. I chose the title "Pieces of History" to use for my consultancy work and I continue to take an interest in the particular subject of artillery.

For those not aware of the pun, I should explain that the word "Piece" has a particular meaning in the world of artillery: a "piece of artillery" is a gun and, although the term is not often used now, it was common practice to refer to guns as "pieces". Since my interest is in historical artillery, the title is now, I hope, clear.

I intend to write a series of short articles about the history of artillery, but I do not intend to write a "history", as such: it would be a major task and one I don't regard as appropriate in a blog! Instead I shall simply pick out odd items that have interested me and post them when the mood takes me. Among these will be the occasional explanation of artillery that seems to be worth putting into this form.

For example, I have often had to explain the importance of trajectories in the context of artillery. Any professional artilleryman has to understand how to shape trajectories, yet for most people, the concept of shaping trajectories sounds weird and somewhat unlikely. Well, perhaps I should tackle that one as my next blog: it could set the tone and allow you to decide whether this is a blog that you want to read!