cted
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.]

~The Use of Lead Cylinders.~--The method of using lead cylinders to test
the strength of an explosive is a very simple affair, and is conducted as
follows:--A solid cast lead cylinder, of any convenient size, is bored
down the centre for some inches, generally until the bore-hole reaches to
about the centre of the block. The volume of this hole is then accurately
measured by pouring water into it from a graduated measure, and its
capacity in cubic centimetres noted. The bore-hole is then emptied and
dried, and a weighed quantity (say 10 grms.) of the explosive pressed well
down to the bottom of the hole. A hole is then made in the explosive (if
dynamite) with a piece of clean and rounded glass rod, large enough to
take the detonator. A piece of fuse, fitted with a detonator, is then
inserted into the explosive and lighted. After the explosion a large pear-
shaped cavity will be found to have been formed, the volume of which is
then measured in the same way as before.

The results thus obtained are only relative, but are of considerable value
for comparing dynamites among themselves (or gun-cottons). Experiments in
lead cylinders gave the rela