Inspired by Canonmn's thread named "Some Interesting Cannon Photos" I dug up some research that I once did on Lieut. Keohler's Depression Carriage design and how it was used at the Siege of Gibralter in 1782. The only question not addressed in any sources I found dealing with firing at steep angles of depression was that of solid shot retention in the tube after loading and before firing. Do any of you guys know how they achieved that?
Before seeing Cannonmn’s photo of the British Royal Artillery Depression Carriage designed by Lieut. Koehler at the Siege of Gibralter, I had no idea that one existed today, although I had seen a sketch of such a unique carriage and had read an account of it’s use several years ago. For those of you who were unaware that such a carriage had been designed and actually put to use, we dedicate the following fascinating details of this depression carriage’s design and historical use during the “Great Siege” which pitted the British at Gibralter against their traditional foes, the French and the Spanish.
“ A few days before an imminent attack by sea, General Elliott was taking a leisurely stroll with two of his aides-de-camp. It was ‘a little after midnight’ and they were discussing various defensive possibilities. Suddenly ‘a ball of fire sprang from behind a certain part of the Rock and fell into the sea’. According to Koehler, Elliott raised ‘his hand with characteristic vivacity’. ’ I accept the omen’, he exclaimed waving his hands ‘like a Roman of the ancient times’. Koelher went on to claim that ‘it was afterwards ascertained that the spot where the meteor first appeared was the site of the batteries that destroyed the ships, and that the spot where it fell was ‘the exact part of the bay in which those ships were moored’.
To explain Lt. Koehler’s unique carriage I took the following from
A history of the Late Siege of Gibralter by John Drinkwater: “The afternoon of the 15
th (Feb. 1782), some practice was made from a gun mounted upon a new-constructed depressing carriage, the invention of Lieut. Koehler, of the Royal Artillery, which was highly approved of by the Governor and other officers present. The Gun as affixed to a bed of timber, the underside of which was a plane parallel to the axis of the piece: from this bed, immediately under the centre of gravity, projected a spindle of eight inches in diameter.
This spindle passed through a groove formed for it’s reception in a plank, the upper side of which was also a plane: upon this under piece the bed and gun recoiled, being attached to it by a key passing through the spindle. The gun and bed by this means were at liberty to move around upon the axis of the spindle, and when fired, slided upon the under plank in the line directed by the groove. He under piece was then connected, by a strong hinge in front, to two cheeks of a common garrison carriage, cut down to be a little higher than the trucks. The gun could be laid to any degree of depression under twenty degrees, by a common quoin resting upon the cheeks of the carriage; but when greater depression was necessary, two upright timbers with indented steps, were fixed to the cheeks; by which, with the assistance of a movable plank, to slide in upon the steps, and a quoin, the back part of the plank, upon which the gun slided, was elevated at pleasure by iron pins in the uprights; and the gun depressed to any angle above twenty and under seventy degrees.
Many advantages, besides that of immediate depression, resulted to the artillery from this invention. The carriage, when the gun was depressed, seldom moved; the gun sliding upon the plank to which it was attached by the spindle, and returning to it’s former place by the most trifling assistance. When the shot was discharged, and the bed with the gun had recoiled to the extremity of the groove; the Matross (sic, an artilleryman who assisted the gunner with loading and maneuvering the gun), by turning round the gun to lie horizontally across the carriage, (which was done with the greatest facility) was also enabled to load under the cover of the Merlon, unexposed to the enemy’s fire, and avoiding the difficulty of ramming the shot upwards.
It equally allowed the gun to be fired point blank, and (by turning the muzzle to the back part of the carriage,) at every elevation, to forty-five degrees, but in that state it(sic)did not particularly excel. As to the accuracy of the depressing shot, no farther proof need be adduced, than that, out of thirty rounds, twenty-eight shot took place (sic) in one traverse in the St. Carlos Battery, at the distance of near one thousand four hundred yards. If the arrangements in the engravings for this work had not been previously established, I should have been happy, by adding a plan of elevation, to have further explained this carriage, which has reflected so much credit upon the ingenious Inventor.”
TracyYou can see some of the unique features of this unusual Depression Carriage in this sketch, which are explained in the complete description of it in Drinkwater’s book.
Spanish and French Floating Batteries such as the ones shown here were used in the Siege of Gibralter which lasted on and off for more than three years. Elevated British batteries on the Rock did a number on them once they found the range.
This was a different type of Floating Battery used, not to challenge the heights, but to duel with the numerous conventional artillery pieces in the King’s Battery at much lower elevation.
An overall view of the siege forces present in the “Great Siege”. 
