MECHANICS AND USEFUL ARTS. 113 



mentioned that the principal difficulty arose from the enormous 

 and hitlierto destructive force of tlie recoil of powerful guns ; and 

 here I shall point out the manner in which that difficulty is over- 

 come. Tliat part of the carriage which is called the elevator may 

 be spoken of and treated as a lever ; tliis lever has the gun-car- 

 riage axle at the end of the power arm, and the centre of gravity 

 of the counter-weight at the end of the weight-arm, there being 

 between them a moving fulcrum. When the gun is in firing 

 jDosition, the fulcrum on which this lever rests is almost coincident 

 with tlie centre of gravity of the counter-wei'ght, and when the 

 gun is fired the elevators roll on the platform, and consequently 

 the fulcrum, or point of support, travels away from the end of the 

 weight-arm towards the end of the power-arm ; or, in other words, 

 it passes from the counter-weight towards the gun. Notice the 

 important result of this arrangement. When the gun is fired, its 

 axle passes backward on the upper or flat part of a cycloid. It is 

 free to recoil, and no strain is put upon any part of the structure, 

 because the counter-weight commences its motion at a very low 

 velocity. As the recoil goes on, however, the case changes com- 

 pletely, for the moving fulcrum travels towards the gun, making 

 the weight-arm longer and longer every inch it travels. Thus the 

 resistance to the recoil, least at first, goes on in an increasing 

 progression as the gun descends, and at the end of the recoil it is 

 seized by a self-acting pawl or clutch. The recoil takes place 

 without any jar, without any sudden strain, and its force is re- 

 tained under the control of the detachment to bring up the gun to 

 the firing position at any moment they may choose to release it. 

 The recoil, moreover, however violent at first, does not put inju- 

 rious horizontal strain on the platform. In my experiments at 

 Edinburgh with a 32-2iounder, I found that so slight was the vi- 

 bration on the platform caused by firing, that the common rails on 

 which the elevators rolled in that experiment, and which were 

 only secured in the slightest manner, did not move from their 

 position, nor even when heavy charges or double charges were 

 used did sand and dust fall off their curved tops. — NostraJicfs 

 Eclectic Engineering. 



GUNPOWDER HAMMER AND PILE-DRIVER. 



At a late meeting of the Franklin Institute, there was exhibited 

 a gunpowder hammer, invented and constructed by Mr. Thomas 

 Shaw, of Philadelphia. In describing the apparatus, Mr. Shaw 

 said : — 



*' A weight or hammer is suspended between vertical guides, and 

 is provided on its under-side with a plunger that fits into the bore 

 of a cvlinder held between the same guides beneath the hammer. 

 It IS intended that the cylinder should rest upon the object to be 

 pounded, and that the hammer should be held by a pawl, which 

 catches into a rack secured parallel with the guides. The pawl 

 is released from the rack by a cord connected with the same, 

 whereupon the hammer is allowed to fall. A small amount of 

 10* 



