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 to have considerable influence.

[Illustration:  FIG. 54.—­MORTAR FOR MEASURING THE BALLISTIC POWER OF EXPLOSIVES. A, Shot; B, Steel Disc; C, Section of Mortar (Cast Iron); D, Wooden Plug holding Explosive (E); F, Fuse.]

Mr T. Johnson made some ballistic tests.  He used a steel mortar and a shot weighing 29 Ibs., and he adopted the plan of measuring the distance to which a given charge, 5 grms., would throw the shot.  He obtained the following results:—­

Range in Feet.