118 Thermotic Effect of Compression and Expansion of Air. 



as to allow the escaping air to expand more freely to the 

 atmospheric pressure. 



Thus the general result may be stated as being, that air 

 being submitted to a pressure of 7 inches of mercury, rose 

 in temperature from 63° to 92°, or 29° Fahr, ; while, in 

 escaping from the same, it fell from 92° to 66°, or 27° Fahr. 



The barometer was not observed on the occasion, but it is 

 believed that 29 6 inches would be extremely close to the 

 height at the time ; and with reference to the index errors of 

 the thermometer, the relative errors of which upon each 

 other are more important than the absolute quantity, but 

 which had been found not to exceed 3° at the freezing point, 

 a comparative trial was made on the evening of the day of 

 experiment. The thermometers were placed side by side in 

 a still room, and after an hour were read off, and both read 

 57°*5 ; they were then put into an adjacent vessel of water, 

 and, after half an hour, being looked at again, each was se- 

 parately found at 56°* 5, so that no sensible quantity of cor- 

 rection was required here. 



The observations themselves — thirty-four in number, taken 

 with all possible precautions to guard against personal errors, 

 by reversing the observers, &c. — are given below, and will 

 aiford a clearer idea of the degree of value to be attached to 

 the result. 



With reference to the practical application of the prin- 

 ciple, this was shewn to be not only possible, but extremely 

 promising, and has been described in the Practical Mechanic's 

 Journal for October 1st, and December 2d, 185Q, while the 

 connection of this isolated fact with the general theory has 

 been taken up by Mr W. Petrie, C.E., in his excellent paper 

 published in the present Number of this Journal, and has 

 also been shewn by Mr Macquorn Rankine, C.E., to be con- 

 formable to the indications of his Mechanical Theory of 

 Heat. 



