CAPTAIN NOBLE AND ME. E. A. ABEL ON EIEED GUNPOWDEE. 
109 
With pebble and other powders, where a slow and tolerably regular combustion takes 
place, the maximum tension of the gas, obtained both by direct measurement and by the 
chronoscope, agrees remarkably closely. There is generally a very slight difference 
indeed between the indicated pressures ; but the case is greatly different where the 
powder is of a highly explosive or quickly burning description. In such a case, not 
only are the pressures indicated by the crusher-gauge generally much above those 
indicated by the chronoscope, but they differ widely in various parts of the powder- 
chamber, in the same experiment, and even in different parts of the same section of the 
bore. They are also locally affected by the form of the powder-chamber, and frequently 
indicate pressures considerably above the normal tensions that would be attained were 
the powder confined in a close vessel. 
It is not difficult to explain these anomalies. When the powder is ignited compara- 
tively slowly and tolerably uniformly, the pressure in the powder-chamber is also uniform, 
and approximates to that due to the density of the products of combustion. 
The crusher-gauges, then, give similar results throughout the powder-chamber, and 
they accord closely with the results deduced from the chronoscope observations. But 
when a rapidly lighting or “ brisante ” powder is used, the products of combustion of 
the portion first ignited are projected with a very high velocity through the interstices 
of the charge, or between the charge and the bore ; and on meeting with any resistance 
their vis viva is reconverted into pressure, producing the anomalous local pressures to 
which we have drawn attention. 
We have pretty clear proof that, when this intense local action is set up, the gases 
are in a state of violent disturbance, and that waves of pressure pass backwards and 
forwards from one end of the charge to the other, the action occasionally lasting the 
whole time that the shot is in the bore. In fact, with the rapidly burning, and in a 
less degree even with the slower burning powders, motion is communicated to the pro- 
jectile not by a steady, gradually decreasing pressure like the expansive action of steam 
in a cylinder, but by a series of impulses more or less violent. 
The time during which these intense local pressures act is of course very minute ; but 
still the existence of the pressures is registered by the crusher-gauges. The chrono- 
scopic records, on the other hand, which are, so to speak, an integration of the infini- 
tesimal impulses communicated to the shot, afford little or no indication of the intensity 
of the local pressures, but give reliable information as to the mean gaseous pressure on 
the base of the shot. 
The two modes of observation are, as we have elsewhere pointed out, complementary 
one to the other. The chronoscope gives no clue to the existence of the local pressures 
which the crusher-gauge shows to exist ; while, on the other hand, where wave- or 
oscillatory action exists, the results of the crusher-gauge cannot be at all relied on as 
indicating the mean pressure in the powder-chamber. 
An interesting illustration of this distinction was afforded by two consecutive rounds 
fired from a 10-inch gun, in one of which wave-action was set up, in the other not. In 
