ions, have shown that the following
units of heat are produced by the detonation of--

Nitro-glycerine 1,784 heat units.
Gun-cotton 1,123 "
Potassic picrate 840 "

which, multiplied by the mechanical equivalent per unit, gives--

Nitro-glycerine 778 metre tons per kilogramme.
Gun-cotton 489 " "
Picrate of potash 366 " "

~Nobel's Ballistic Test.~--Alfred Nobel was the first to make use of the
mortar test to measure the (ballistic) power of explosives. The use of the
mortar for measuring the relative power of explosives does not give very
accurate results, but at the same time the information obtained is of
considerable value from a practical point of view. The mortar consists of
a solid cylinder of cast iron, one end of which has been bored to a depth
of 9 inches, the diameter of the bore being 4 inches. At the bottom of the
bore-hole is a steel disc 3 inches thick, in which another hole has been
bored 3 inches by 2 inches. The mortar (Fig. 54) itself is fitted with
trunnions, and firmly fixed in a very solid wooden carriage, which is
securely bolted down to the ground. The shot used should weigh 28 lbs.,
and be turned accurately to fit the bore of the mortar. Down its centre is
a hole through which the fuse is put.

The following is the method of making an experiment:--A piece of hard wood
is turned in the lathe to exactly fit the hole in the steel disc at the
bottom of the bore. This wooden cylinder itself contains a small cavity
into which the explosive is put. Ten grms. is a very convenient quantity.
Before placing in the mortar, a hole may be made in the explosive by means
of a piece of glass rod of such a size that the detonator to be used will
just fit into it. After placing the wooden cylinder containing the
explosive in the cavity at the bottom of the bore, the shot, slightly
oiled, is allowed to fall gently down on to it. A piece of fuse about a
foot long, and fitted with a detonator, is now pushed through the hole in
the centre of the shot until the detonator is embedded in the explosive.
The fuse is now lighted, and the distance to which the shot is thrown is
carefully measured. The range should be marked out with pegs into yards
and fractions of yards, especially at the end opposite to the mortar. The
mortar should be inclined at an angle of 45 deg.. In experimenting with this
apparatus, the force and direction of the wind will be found