Finally Berthelot’s theoretical calculations give a specific pressure of—­

Powder                   1
Dynamite                13
Gun-cotton              14
Nitro-glycerine         16
Blasting gelatine       17

It will be observed that the practical results vary largely from the theoretical values, but they seem to indicate that gun-cotton and No. 1 dynamite are very nearly equal to each other, and that in the nitro-glycerine compounds, except where gun-cotton is added, the force appears to be nearly in proportion to the nitro-glycerine contained.  From the foregoing it seems fair to estimate roughly the values of bursting charges of shells as follows: 

Powder                      1
Gun-cotton and dynamite     6 to 10
Nitro-glycerine            13 to 15
Blasting gelatine          15 to 17

Attention has been turned in Europe for more than thirty years toward firing high explosives in shells; but it is only within very late years that results have been reached which are claimed as satisfactory, and it is exceedingly difficult to obtain reliable accounts even of these.  Dynamite was fired in Sweden in 1867 in small quantities, and a few years later it was fired in France.  But two difficulties soon presented themselves.  If the quantity of nitro-glycerine in dynamite was small, it could be fired in ordinary shells, but the effect was no better than with gunpowder.  If the dynamite was stronger in nitro-glycerine, it took but a small quantity to burst the gun.

As early as 1864, dry gun-cotton was safely fired in shells in small quantities, but when a sufficient quantity to fill the shell cavity was used, the gun burst.  Some few years ago it was found that if the gun-cotton was either wet or soaked in paraffin, it could be fired with safety from powder guns in ordinary shells, provided the quantity was small in proportion to the total weight of the shell—­say five or six per cent.  But a new difficulty arises from the fact that it breaks the shell up into very small pieces, and it is an unsettled question among artillerists whether more damage is done to an enemy by breaking a shell into comparatively large pieces and dispersing them a long distance with a bursting charge of powder, which has a propulsive force, or by breaking it with a detonating compound into fine pieces, which are not driven nearly so far.  When used against troops there is also the objection to the high explosive shell that it makes scarcely any smoke in bursting, and smoke at this point is useful to the artillerist in rectifying his aim.

In the matter of shells for piercing armor, however, there are no two opinions regarding the nature of the bursting charge.  To pierce modern armor at all a shell must be made of forged steel, so thick that the capacity of the cavity for the bursting charge is reduced to one-fourth or one-fifth of what it is in the common shell; the result is that a charge of powder is frequently not powerful enough to burst the shell at all; it simply blows the plug out of the filling hole in the rear.  In addition it is found that in passing through armor, the heat generated is so great that the powder is prematurely ignited.