MECHANICS AND USEFUL ARTS. 51 



force generated by the explosion of gunpowder, a point on which artillerists 

 are as yet very far from being completely agreed. Robins, who first at- 

 tempted its determination, valued it at one thousand atmospheres, or nearly 

 seven tons on the square inch; Hutton, who endeavored to ascertain it by 

 meiins of the ballistic pendulum, estimated it at from thirteen to seventeen 

 tons; and Captain Boxer, whose method consists in measuring the actual 

 bulk of permanent gas evolved by the combustion of a known amount of 

 gunpowder, arrives at the result of rather more than twenty-two tons. Inde- 

 pendently of the great discrepancy between these several estimates, Mr. 

 Longridge declines to receive them, on the ground of the inaccuracy of the 

 methods by which they have been made. In the last method, especially, 

 there appears to be several sources of error; for not only has the heat gener- 

 ated by the combustion of gunpowder never been experimentally determined, 

 but, it is also quite possible that the expansion of gases at so extreme a 

 temperature may not altogether be regulated by the law of Mariotte. 

 ^Further, it presupposes the instantaneous com-ersion of the whole of the 

 powder into gas. Mr. Longridge accordingly instituted a series of experi- 

 ments of his own, based upon the determination of the amount of gunpowder 

 required to burst a cylinder of known strength, from which he concluded 

 that the ultimate force of the powder used government powder did not 

 exceed seventeen tons per square inch. This amount of force can never, he 

 says, be permanently resisted by a gun made of cast iron, a material whose 

 tensile strength is estimated at not more than eight tons per square inch, 

 especially if, as is generally the case in England, the gun be cast solid, and 

 subsequently bored, since the unequal rate of cooling of the inner and outer 

 parts cannot fail to produce serious flaws in its mass. All the money, there- 

 fore, which is now being spent in rifling cast-iron ordnance is simply thrown 

 away. The same objection applies to the construction of a gun by a single 

 casting from any homogeneous material whatever. The only way of attain- 

 ing the maximum of strength is, to build up the gun, layer by layer, in such 

 a manner that each successive layer, from within outward, shall be in a 

 progressively increasing state of tension. It is on this principle that the 

 guns of Sir W. Armstrong, Mr. \Vhitworth, and Captain Blakely are made. 

 The method of carrying it out, however, adopted by all these gentlemen, 

 consists in encircling a central tube, of various material, with successive 

 rings of iron, which are either shrunk on by cooling, or forced on when cold 

 by hydraulic pressure. This mode of operation can never, says Mr. Long- 

 ridge, lead to perfectly satisfactory results. The extreme nicety with which 

 the tension of each successive layer ought to be regulated the deviation of 

 one five-hundredth of an inch from the required size being sufficient to 

 materially impair the strength of the gun can never be arrived at by the 

 contraction of a heated ring; and the rings, however put on, must sooner or 

 later be loosened by the repeated shocks of the explosion. The plan proposed 

 by Mr. Longridge is, to wind round a central tube successive spiral layers of 

 steel wire, until the desired strength is attained, the greatest attention being 

 paid to the exact tension of each successive layer. He does not enter at all 

 into the question of what is the best material for the central tube. On the 

 contrary, his sole object being to exhibit in the most striking light 

 the immense power of resistance given by the layers of wire, he appears 

 purposely to have made the core as weak as he well could. The results of a 

 series of private experiments on a small scale were so encouraging that he 

 constructed a brass cylinder, of about three inches bore and a yard long, 



