THE PLOW OF GASES. 179 



the value of K, the coefficient of contraction, given by 

 Weisbach {' The Steam Engine/ p. 324, Rankine) is from 

 •73 to '833, Whether these are the real coefficients of 

 contraction may, however, well be doubted, as it is ex- 

 tremely difficult to determine the experimental quantities 

 of gas discharged owing to the great eflFect of slight varia- 

 tions of temperature on the relations between changes of 

 pressure and changes of temperature, such changes of tem- 

 perature being almost necessarily incidental on changes of 

 pressure. 



II. In Mr.Wilde^s experiments the pressure was allowed 

 to fall in the discharging vessel during the discharges ; 

 this would cause a corresponding fall of temperature, 

 which would again cause heat to flow from the metal 

 vessel into the gas within. 



It is difficult therefore to say what the change of tem- 

 perature was except in the extreme cases. With the 

 experiments on the highest pressure, however, the times 

 7*5 seconds, and the greatest possible falls of temperature 

 5°'5, were so small that the communication of heat from 

 the walls of the receiver would have been very slight ; and 

 hence we might expect that the discharges, calculated on 

 the assumption of no communication of heat, would agree 

 with the theoretical discharges multiplied by the real 

 coefficient of contraction. This would be shown by an 

 agreement in the successive coefficients obtained from the 

 experiments with the higher pressures. On the other 

 hand, with the lowest pressures the times were so con- 

 siderable, 170 seconds, and the greatest possible falls of 

 temperature (assuming no conduction, 94°) so great, that 

 the communication of heat would have been very great 

 and, considering the comparatively small mass to be 

 heated (only one thirteenth of what it is in the highest 

 experiments), might maintain the temperature approxi- 



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