The eccentric engineer: Koehler’s genius design of the depressing carriage

In the early days of September 1782, the largest action of the whole of the American War of Independence was about to start. Despite the battle of Yorktown, which sealed US independence, being nearly a year before, the war was not quite over, and this revolutionary battle would have a number of unique features. Firstly, not a single American was taking part, and secondly, it was happening not in the New World, but around Gibraltar in the Mediterranean.

When the Revolutionary War broke out, the Americans had found themselves with some unexpected allies in the form of the ‘Bourbon Alliance’, a combination of French and Spanish powers whose interests had as much to do with containing the British as with freeing American colonists. So, with the fighting in America all but over, the biggest engagement of the war had yet to begin.

This gave George Eliott, the British Governor defending Gibraltar, something of a problem. He had been besieged by the Bourbon allies since June 1779, but the Royal Navy had always managed to run the naval blockade and resupply his garrison. Now the Bourbons, realising they were unlikely to starve the garrison out, had decided on a full frontal attack, ranging 65,000 troops, 47 ships and 10 floating batteries against Eliott’s 7,500 men and 12 gunboats.

Of course, anyone who knows Gibraltar can tell you that Eliott had one huge advantage. The Rock, standing high above the Spanish plain, was a fortress and had also been heavily fortified. While the Spanish and French had remained far back on their own lines, this gave the British a clear line of fire, but with 65,000 troops now approaching from land and sea, Eliott had a problem – his guns.

Eliott’s cannons were mounted on the usual garrison carriages, with a rectangular frame mounted on four small wheels or ‘trucks’. There were great for firing horizontally, as in a naval broadside, but once the enemy were too close to the bottom of the Rock, they became useless.

It was possible to wedge the carriages so the guns could point down the cliff face, but this had two serious drawbacks. First, the gun had to be fixed in full view of the enemy and could not be withdrawn for reloading. Second, when the gun was fired at this angle, the recoil made the whole carriage leap in the air, damaging the trucks, guns and the men trying to fire them.

It was at this point that Lieutenant George Frederick Koehler of the Royal Artillery came to the rescue. Koehler was an artist and engineer whose father had emigrated to England from Bavaria. Having found himself caught up in the siege, he had put his mind to solving Eliott’s gun issue and finally presented the Governor with ‘Koehler’s Depressing Carriage’. While this might have sounded like a vehicle for taking the defeated Eliott from his doomed fortress, it was, in truth, its saviour.

Koehler’s depressing carriage was a gun mount that allowed cannons to fire downhill. He was not the first person to come up with that idea, but his combination of two unique systems made it the first to work.

The first element of Koehler’s design was to hinge the front of the carriage on an iron spindle, allowing the gun to be pointed downwards. Provided the cannon was loaded horizontally and heavily wadded to prevent the charge and ball from rolling out, the gun could then be aimed down at nearly 50 degrees.

Then, to solve the recoil problem. Koehler invented a sliding bed on which the cannon sat, allowing it to slip backwards and forwards. When the cannon was fired, the gun shot back on the bed but the carriage itself remained still. This simple device proved to be the forerunner of the recoil system used on just about every artillery piece since.

This ‘depressing carriage’ also boasted two other benefits. As the carriage didn’t need to be strapped in place, the gun could be reloaded in the safety of the casemates and, should the defenders ever need to shoot uphill, the cannon would simply be reversed on the carriage allowing an upward firing position of 45 degrees.

Of course, as with all great military development, the proof comes in the battle. After over three years of siege, the ‘Grand Assault’ of the French and Spanish began on 13 September 1782 and proved a disaster for them. With the enemy driven back from the Rock, Admiral Howe was able to fully resupply the garrison and the French and Spanish finally lifted the siege., ending the last action in the American Revolutionary War.