* 



156 W. E. Woodbridge on the Pressure of Fired Gunpowder. 



parts of the graduated tube successively in view. The receiver 

 was connected by tubes to a compressing pump and to a pressure- 

 guage. The pressure was measured by weights suspended so as 

 to press directly on the valve of the gauge, over its centre, the 

 relation ^between the pressure per square inch and the weight on 

 the valve having been previously determined by a comparison 

 with the pressure of a column of mercury fifty-two feet in height. 



The precautions for the safety of the observer consisted in 

 viewing the progress of compression through the strong plate 

 glass eyes of a mask, and a small aperture in a plate of iron, 

 interposed between them and the windows of the receiver. 



The compression at pressures above 10,000 lbs. per square inch 

 was ascertained by the use of the steel piezometer enclosed in a 

 receiver of cast-steel, the motion of the piston being registered 

 by the mark on the stem, as already explained. 



At a pressure of 10,000 lbs. per square inch and temperature 

 of 60° Fahr., the apparent compression of the oil, (disregarding 

 that of the glass,) was 0*03059 its original volume being 1. At 

 lower pressures, the compression indicated was nearly propor- 

 tional to the pressure applied, though its rate decreases some- 

 what as the latter increases. This modification continues when 

 the pressure is above 10,000 lbs. per square inch, but before it is 

 raised to 20,000 lbs. per square inch, the degree of compression 

 augments more rapidly than the pressure. 



« At 50° the. compression of the oil was less regular in its ratio 

 to the pressure employed, being greater as that was increased — 

 suggesting the idea of the solidification of some of the more 

 easily congealable portions of the oil. 



To state at length all the considerations relative to the appli- 

 cation of the piezometer which has been described, to the 

 measurement of the pressure of fired gunpowder would extend 

 this paper too far. It is however necessary to mention the influ- 

 ence of the change of temperature consequent on rapid conden- 

 sation upon the amount of compression produced by any given 

 force, the only circumstance, probably, modifying in an appreci- 

 able degree the correspondence between the pressure indicated 

 by a stroke of the piezometer produced by slow compression, 

 and that indicated by a mark of equal length produced by the 

 action of fired gunpowder. When the compression is very 

 slowly conducted, the change in the specific heat of the oil due 

 to its condensation effects no observable alteration in its temper- 

 ature, for it readily imparts its surplus heat to the bodies with 

 which it is in contact. But if the compression be effected sud- 

 denly, any decrease in the specific heat of the liquid must be 

 accompanied by a corresponding rise of temperature, and the 

 compression produced in the latter case will be less, by the 

 amount of the expansion which would, under that pressure, be 



