May 19, 1887] 



NATURE 



59 



at an uncertain rate, but the moment the shot leaves the 

 gun the whole of the products of combustion appear to 

 spring out with a velocity equal to, if not greater than, 

 that of the shot. The evidence of this supposition is 

 found in the fact that in the case of disappearing guns 

 fired with their muzzles close to a masonry parapet, and 

 in which the recoil below it is completed in a small fraction 

 of a second, no blackening of the masonry is noticeable. 

 A portion of the gases follow the shot and keep up with 

 it for a considerable distance, as is shown by the circum- 

 stance that smoke issues plentifully from the earth banks 

 into which proof shots at short range are fired, proving 

 that the smoke of the discharge must have followed the 

 shot into the tunnel momentarily made and as quickly 

 obliterated by passage of the projectile through the earth. 

 It is evident that the velocity with which the gases issue 

 must depend upon the pressure in the gun at the moment 

 of the shot leaving the muzzle, and this pressure again 

 depends upon the volume of the bore, the weight of 

 powder consumed, and the final temperature, the latter 

 depending partly upon the expansion and the consequent 

 heat converted into work. 



The final temperature of the gases can only be conjec- 

 tured : it probably does not exceed a bright red heat, or 

 between 1200" C. and 1400' C. absolute; and knowing 

 that one pound of powder at 0° C. and standard baro- 

 meter develops about 4'48 cubic feet of gas, it is possible 

 to estimate what the final pressure in the gun should be. 

 Given, however, a barrel full of gas at a definite pressure, 

 we are not in a condition to say what energy its expulsion 

 would generate ; and the assumption that the mean 

 velocity will be that due to a body falling from a height 

 equal to that of a column of gas of uniform maximum 

 density which would correspond to the observed pressure 

 would probably be as accurate as any other. On that 

 assumption the velocity of the gas would be 4544 times 

 the square root of the product of the final pressure in the 

 bore in tons per square inch into its volume in cubic feet 

 divided by the weight of the powder in pounds, and, this 

 velocity determined, the energy is, of course, at once 

 arrived at. 



The displacement of the atmosphere also forms a 

 very considerable item. The expansion on leaving 

 the gun being instantaneous, the pressures and tem- 

 peratures fall approximately as in adiabatic expansion ; 

 hence it is easy to calculate what probable temperature 

 and consequent volume the gases will assume as they 

 stream out of the gun, and this temperature is com- 

 paratively low ; otherwise powder smoke would be in- 

 supportable to those feeling its influence close to a gun. 

 The work done in displacing the air is found by multi- 

 plying the volume pushed aside by the atmospheric 

 pressure. A small portion of this work is performed as 

 the shot travels along the chase, but the greater part is 

 done after it leaves the muzzle. The energy of the reac- 

 tion to the sudden liberation of gases under high pressure 

 is but too familiar to us in the case of boiler explosions, 

 in which it commonly happens that great masses of 

 material are hurled with destructive force and often to 

 great distances. 



The pressure-curve inside the gun is still very ill- 

 defined ; the forms commonly given are certainly a long 

 way from the truth, because the areas included, which 

 form indicator diagrams representing the work done, 

 will not account for the energy developed. The pressure 

 probably falls in proportion to the distance travelled by 

 the shot, and the time in which the discharge takes 

 place may be calculated on that assumption, or even 

 with sufficient accuracy on the supposition that the 

 velocity of the shot is uniformly accelerated as from 

 the action of a constant force equal to the mean pressure 

 producing the known velocity of the shot in a known 

 distance. The easiest way to take account of all the 

 forces causing recoil is to ascertain the velocity of the 



combined powder and projectile which will possess the 

 total energy of discharge calculated, and then to equate 

 the momentum of the gun and moving parts of the 

 carriage to that of the shot and powder. In the case of 

 a carriage receding along a slide, this operation is a very 

 easy one, but when a Moncrieff mounting has to be dealt 

 with, the case becomes very complicated, the gun moves 

 along a curved path, the sides and counterweights have a 

 rolling motion, and it becomes necessary to calculate the 

 path of the centre of gyration, and determine the virtual 

 weight concentrated in the gun, and a similar process 

 has to be followed in the case of the massive levers which 

 carry the guns in hydro-pneumatic mountings. 



Recoil consists of two parts : first, the period, a very 

 brief one, in which the velocity of recoil is got up ; and 

 secondly, the period in which the energy so acquired by 

 the parts in motion is more slowly absorbed or dissipated. 

 The first part of recoil must necessarily occupy the same 

 time as the discharge, that is to say, a small fraction of a 

 second, because acceleration can only go on so long as 

 the accelerating force is acting, but that force is the 

 pressure of the powder gases on the base of the bore, and 

 the pressure only lasts while the discharge is taking 

 place. The motion of the whole system of gun and 

 carriage does not, however, coincide with the motion of 

 the shot. In all but very long guns the shot has left the 

 barrel before the motion of the muzzle commences ; during 

 the time of discharge, perhaps the 1/50 part of a second, 

 the gun is being stretched loy the inertia of its forward 

 end and of the carriage resisting the tendency to put 

 them into motion, but the reaction to this stretching 

 carries on the acceleration of recoil a little after the 

 shot has left the gun. The pressure on the parts 

 during this period is very severe, the work done being 

 exactly the same as that performed by the shot, the 

 powder, and the displacement of the atmosphere. The 

 full speed of recoil is attained, not only in a very 

 short time, but in a very restricted space, rarely more 

 than 3 inches, and the difficulty in constructing carriages 

 may be said to lie in providing for the violent strains 

 which produce a velocity of some 20 feet a second in the 

 great mass of the gun and carriage in the exceeding 

 short time and space named. The momentum of the 

 moving parts of the system being equal to that of the 

 ejected charge, the velocity is readily calculated, and 

 generally ranges between 16 and 30 feet per second, and 

 their energy is then easily ascertained. 



In the counterweight Moncrieff carriages, which have 

 been made for short muzzle-loaders up to 9-inch calibre 

 and twelve tons weight, the whole mass set in motion is 

 so great compared with the energy of the discharge, that 

 the gun sinks below the parapet with a comparatively 

 slow and stately movement ; but, with the long breech- 

 loaders and heavy charges, with the comparatively light 

 moving parts which characterise hydro-pneumatic mount- 

 ings, the motion is very violent, and requires great 

 strength in the parts to resist the strains. In addition, 

 the gun describes a circular path, and by the time the 

 maximum velocity of recoil is attained, sufficient centri- 

 fugal force is engendered to produce a sudden upward 

 pull, which has to be met by arrangements for holding 

 the carriage down to the masonry of the emplacements. 

 The longer the arms which cany the gun, the less this 

 tendency is, because the pull of centrifugal force is in- 

 versely as the length of the radius of the curve described 

 by the trunnions. The front of the carriage has generally 

 to be held down for another reason. The gun, when fired, 

 is high above the base of the mounting ; the mechanism, 

 self-contained in the carriage, for absorbing recoil, offers 

 a certain amount of resistance to the backward move- 

 ment of the gun, hence a couple is established which 

 tends to turn the carriage over on its rear wheels, and 

 this tendency varies with the height of the gun and the 

 length of the base. 



