from Air 'when it undergoes a given Condensatio7i. 93 



air and the atmosphere : the condensed air will rush out and 

 expand within the vessel, attended with a decrease of pressure, 

 an absorption of heat and an equal depression of temperature ; 

 and the last equation will now assume this form, viz. 



We must here conceive that Sp' and Ai vary together, and 

 in a very short space of time the pressure will have decreased 

 to its original quantity p : at the instant this is observed to 

 take place, the communication with the external air must be 

 shut, and then we shall have. 



But this state of the air will be momentary only ; for the loss 

 of temperature will be supplied, and the pressure will increase 

 a little : let p" be the pressure when it is observed to be sta- 

 tionary, then finally, 



Now by comparing ('t) with (3) and (2), we get, 



-4 = 1 - a A ?, 

 P 



Taking the two experiments, one by MM. Clement and Des- 

 ormes, and the other by MM. Gay-Lussac and Welter, of 

 which the particulars are given in the Mecanique Celeste* , we 



find — = 0*354 from the first, and -^ = 0*3724; from the 



second. By the two latter philosophers the experiment was 

 repeated in a great variety of cii'cumstances, the pressure be- 

 ing varied from 144''°"' to 1460""°, and the temperature from 

 —20° to 40° of the centigrade thermometer; and the results 

 were found nearly the same in every case, and upon the whole 

 equal to about 0'3748, or 0*375 = f. This experiment was 

 contrived expressly for solving the problem concerning the 



velocity of sound; for -s/l + -j is the factor by which, ac- 

 cording to the suggestion of Laplace, the velocity determined 

 by Newton's Theory must be multiplied, in order to get the 

 true velocity. When a method for finding the numerical 



• Liv. xii. chap. 3. 



value 



