3^6 OF GUNnMETAL^

conclusive in all cases, though made with the utmost accuracy;as there may be defects in the mass of metal, for whichallowance cannot be made. To avoid such a tedious process,the scale of pressures of the fluid against the shot' may be’constructed; this method of resolving the problem is the mosteasy and simple: but the greater the distance of the pointwhere the thickness of metal is determined from the placewhere the (hot is lodged in the gun, the less exact will be thesolution.

83. To enable guns to resist the pressure of the fluid, thethicknefles of metal mbst be determined from the scale ofpressures. With respect to heavy artillery (64) several roundsshould be fired from three or four guns, differing in lengthbut of the same calibre as the guns whose thicknesses aresought, with the larger service charges; the wads rammeddown with mere force than with the medium charge; andthe initial velocity of each (hot measured. (Treatise on Pow-der.) To construct a scale for estimating the proper thick-nefles of medium artillery (71), the largest charges shouldbe used that the guns will bear, and the wads rammed withthe same force as the wads of guns more reinforced and loadedwith the common charge: it is essential to observe this, asfrom the different resistance of the wads, a considerable diffe-rence arises in the ordinate that exprefles the greatest pressureof the elastic fluid, and successively in the other ordinates; asmay be inferred from the fourth chapter of the Treatise onPowder, where the modifications of the elastic fluid in thecylindric bores of guns are considered. The initial veloci-ties of the (hot being measured, and a line drawn to expressthe scale of velocities in certain spaces, the scale of corre-sponding pressures may be found from this line by the me-thod directed in the treatise on moving bodies. It should beremarked that the velocities of shot from guns of large calibrefired with large charges, are produced by the pressure andimpulsion of the elastic fluid, which, generated at the bot-tom of the bore, acts with such force on the (hot (37) as tocommunicate to it a constant velocity, which the second wadhowever strongly it may be rammed cannot destroy. Thescale should then be parallel to the axis of the gun ; and itsdistance from it equal to a right line drawn perpendicularlyto the axis, expressing the constant velocity produced by theimpulsion of the. fluid,

84. TIu^

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