1895.] Measurement of Pressures by the Crusher- Gauge. 405 



which the gas was kept in the vessel being taken as the pressure 

 required. A pressure by copper- crusher was taken in each experi- 

 ment made, and the mean of the series taken ; the compression of 

 the crusher was the same in the experiments of a series, whether 

 the gases blew out of, or were kept in, the explosion-vessel, the 

 weights apparently not being raised before the compression of the 

 copper had come to an end. 



It may be not superfluous to mention that the copper-crusher is a 

 small cylinder of copper, before compression 0'5 in. long and 

 O326 in. in diameter, one end of which rests on the base of a steel 

 case in which it is contained, the other end being in contact with one 

 end of a movable piston of 0'461 in. diameter (i sq. in. section), on the 

 other end of which the gases exert their pressure ; suitable provision 

 is made to prevent entrance of gas to the gauge-case, and to hold 

 the copper-crusher in position while allowing room for its widening 

 in the middle by compression. The coppers are used previously 

 pressed to about a ton short of the pressure to be measured. 



Rumford, in 1797, used a " weights " method to ascertain the rela- 

 tion existing between the pressure produced by a fired gunpowder 



and the density of the charge ( "eight of gunpowder \ He ^ 



\volume of explosion- vessel/ 



charges of gunpowder from 1 to 18 grains in weight, density of 

 charge varied from 0*04 to 0'76, and the pressures measured were 

 consequently very high ; the weight just lifted by the pressure of the 

 powder gases was found by a series of trials. 



Bunsen also (* Grasometrische Methoden ') used the same method, 

 of course very much altered in details, to ascertain the pressures 

 produced by the combustion of some gases in oxygen ; the gaseous 

 mixtures were at 1 atmos. pressure, the increase of pressure pro- 

 duced on firing the mixtures was at highest about 10 atmos. (about 

 Jg- ton per sq. in.). 



In our experiments, the gases of the fired explosive served only as 

 a medium for comparing the two methods of estimating pressure, so 

 that the question of any lowering of temperature by heating of the 

 explosion- vessel did not come into consideration. 



This direct experimental method of checking the crusher-gauge 

 indications is preferable to calculation of pressure from the ascer- 

 tained amounts of permanent gases, water, and quantity of heat 

 produced at the pressures in question, because the calculation requires 

 knowledge of the specific heat of the gases of combustion at (in the 

 case of cordite) about 3000 C., and although there is valuable 

 information on the subject, it is scarcely sufficiently accurate for this 

 application of it. Again, the compression of the copper-crusher 

 measures the maximum pressure of the gases, when (at the tempera- 

 ture mentioned) some dissociation of carbonic acid would occur (or 



