The instrument is used in the following manner:—­A plug of lead 1 inch long and 1 inch in diameter, and of a cylindrical form, is placed upon the steel plate between the uprights a, the piston placed upon it, the carefully weighed explosive placed in the cavity, and the shot lowered gently upon the piston.  A piece of fuse, with a detonator fixed at one end, is then pushed through the hole in the shot until it reaches the explosive contained in the cavity in the piston.  The fuse is lighted.  When the charge is exploded, the shot is thrown out, and the lead cylinder is more or less compressed.  The lead plugs must be of a uniform density and homogeneous structure, and should be cut from lead rods that have been drawn, and not cast separately from small masses of metal.

[Illustration:  FIG. 56.—­b, STEEL PUNCH; c, LEAD CYLINDER FOR USE WITH PRESSURE GAUGE.]

The strength of the explosive is proportional to the work performed in reducing the height of the lead (or copper) plug, and to get an expression for the work done it is necessary to find the number of foot-pounds (or kilogrammetres) required to produce the different amounts of compression.  This is done by submitting exactly similar cylinders of lead to a crushing under weights acting without initial velocity, and measuring the reduced heights of the cylinders; from these results a table is constructed establishing empirical relations between the reduced heights and the corresponding weights; the cylinders are measured both before and after insertion in the pressure gauge by means of an instrument known as the micrometer calipers (Fig. 57).[A]

[Footnote A:  An instrument called a “Foot-pounds Machine” has been invented by Lieut.  Quinan, U.S.  Army.  It consists of three boards, connected so as to form a slide 16 feet high, in which a weight (the shot of the pressure gauge) can fall freely.  One of the boards is graduated into feet and half feet.  The horizontal board at the bottom, upon which the others are nailed, rests upon a heavy post set deep in the ground, upon which is placed the piston of the gauge, which in this case serves as an anvil on which to place the lead cylinders.  The shot is raised by means of a pulley, fixed at the top of the structure, to any desired height, and let go by releasing the clutch that holds it.  The difference between the original length and the reduced length gives the compression caused by the blow of the shot in falling, and gives the value in foot-pounds required to produce the different amounts of compression. (Vide Jour.  U.S.  Naval Inst., 1892.)]

[Illustration:  FIG. 57.—­MICROMETER CALIPERS FOR MEASURING DIAMETER OF LEAD CYLINDERS.]