OF GUNS.
157
section multiplied into q — the tenacity of the metal will givecm q for the resistance of the female screw. The formula 9962ns, expresses the pressure of the elastic fluid against a superficies= S. I hen if half the diameter B D = r, and its circumference
= C, S = — will be the superficies of the circle, against2
which the elastic fluid acts in the direction F G; then 9962
n x —, will express the force with which the powder acts
against the base of the bouch. For example, if under cir-cumstances the least favourable to the resistance of the bouch,
r =—, n— 1200 (33) and the metal of a less tenacious qua-4
lity be heated as much as it can be on service, mq = 7358758( 35 ) the resistance of the bouch. Now supposing m— \ ;then 4981 nr~ 4981x1,200X5=1444300 for the actionof the powder against the base of the bouch : thus the resist-ance of the female screw will be more than five times theforce that tends to drive the bouch from its place; an equi-librium between the propelling and resisting powers will bethen fully established, if only part of the bouch QR=i ofD Q_be spirated.
127. When the base of the bouch corresponds with theupper part of the bore, the action of the powder against it isas the pressure : but when the bouch only reaches to L R,and leaves the vacuum DLRB, then the elastic fluid actsagainst it with so much the greater force, as L R is distantfrom the upper part of the bore; and greatly exceeds thepressure that it would exert against the base, were the bouchin the former situation. The reflections in the 2 nd chapteron the orbicular cavities formed in the bores of guns cast ofsoft metal, by the impulsion of the elastic fluid, will convey ajust idea of this force.
128. To shorten the operation of making the screws,and at the same time to obtain a resistance superior to theforce of the powder, the bouch may be made in the follow-ing or any similar manner. (PI. 4, Fig. rz) Let it be spi-rated only from P to L; let P L be about £ of F G and theremainder LF be in a form of a truncated cone H K N H,the diameter H H being less than B D, to form a ledgeB H H D, upon which may be placed a thin circular plate of
ductile