THE NATURE AND 



[SECT. ii. 



To determine the pressure at higher temperatures with the apparatus, the end K 

 of the tube M N K was inserted at E ; and as the temperature increased, the 

 pressure of the steam in the cistern L caused the mercury to ascend, and con- 

 sequently afforded a means of measuring the amount of expansive force. 



The objection to this mode of trial is, that the temperature of the mercury must 

 be continually changing during the trial, and steam must be either condensing or 

 generating on its .surface during the time of observation. At each observation the 

 temperature of the whole of the apparatus ought to be the same, and then the 

 column exhibiting the pressure ought to be reduced to its equivalent at the mean 

 temperature. The only observation where these circumstances would have place 

 was that which appears to have been made when the thermometer was at 42 ; 

 then the column in the syphon was 29'7, and the barometer stood at 29'84 : the 

 difference is the force of steam at 42, and is 0-14 inches. By cooling down to 32 

 the force was not perceptibly different, and we know from later trials that this is 

 nearly correct. Professor Robison, however, seems to have thought it was neces- 

 sary to have the force at 32 . 1 



ROBISON'S EXPERIMENTS ON THE FORCE OF STEAM. 



If the elastic force '14, from which Robison began to register, had been added to 

 all the experiments below 212, as it ought to have been, they would have agreed 

 extremely near with the results of later experiments. The experiments made by 

 Achard seldom vary more than a degree or two from those in the above table. 



96. Mr. Dalton's inquiries were conducted by a different method. He took a 



1 Mechan. Phil. vol. ii. p. 36. 



