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which is rarer ; but then the maffes muft be unequal ; for while 

 they remain the fame the weights muft be equal, and confe- 

 quently fo muft the heights to which they elevate the mercury. If, 

 therefore, an alteration of temperature alters the height of the 

 barometer, it muft be by diminilhing or encreafing the mafs of the 

 atmofphere. Now it appears by obfervation, that a variation of 

 the mafs of the atmofphere is not a neceflary confequence of an 

 alteration of the temperature ; for the ^ is often at the fame 

 height at different feafons, and at different places in the fame 

 feafon ; for inftance, in winter, at London and Peterfburgh, 

 though the temperatures at thofe feafons and places be very 

 different ; and even when the height of the ^ changes fimul- 

 taneoufly with the temperature, the change is often diredlly con- 

 trary to that which the change of temperature would lead one 

 to cxped. Thus on the 9th of January, 1777, at eight o'clock 

 in the morning, the thermometer at London flood at 19° under 

 a N. wind, and the barometer at 29,69 ; but at two o'clock in 

 the afternoon, the wind being W. by S. the thermometer rofe to 

 31,5, and the barometer to 29,7, inftead of falling, as might be 

 cxpeded, from an encreafe of 12,5 degrees of heat. Phil. Tranf 

 1778, P- 574- 



Besides, great changes of temperature take place only in the 

 lower atmofphere ; in the higher regions they are inconflderable. 

 Now any increment or decrement of the mafs of the lower 

 atmofphere that can be afcribed to a change of temperature is 

 too fmall to produce any confiderable alteration in the height of 

 the barometer ; for in winter the height to which any confider- 

 able variation of temperature may be fuppofed to extend, 



fcarcely 



