Of BAROMETRICAL MEASUREMENTS. 93 



when the mercury in the barometer flands at the height b. But 

 this formula cannot be juft, otherwife air, comprefled by no 

 greater a force than that of 4.5 inches of mercury, would be 

 incapable of dilatation by heat, or contraction by cold. 



8. IT will agree equally well with the experiments, and will 

 involve no contradiction, even in the extreme cafes, to fuppofe, 

 that the expanfion for a certain degree of heat is as a certain 

 power of the compreffing force. If this power be called /<*, m 

 being the expanfion for i degree of heat, when the mercury in 

 the barometer is of the height l>, the expanfion for any other 



height of the mercury, as P, will be m ; and combining this 



b 



with the former formula for expanfion ( 5.), we have the fpace 

 which air occupies, as far as it depends on temperature, 



32 h 



(j 21) From -a compari'fon of General ROY'S expe- 

 riments *, ju appears to be between ~ and -f ; and it mufl be 

 confefTed, that it is very difficult to affign its value within near- 

 er limits. The form of the correction, however, if not its 

 abfolute quantity, may be found from what is here determined. 

 The lafl of thefe mufl be afcertained by future experiments. 



9. THESE inequalities belong to the temperature of the air ', 

 there is another that depends wholly on the compreffion. In 

 deducing the rule for the meafurement of heights by the baro- 

 meter, it has hitherto been fuppofed,. agreeably to the experi- 

 ments of Mr BOYLE and M. MARIOTTE, that the denfity of the 

 air, while its temperature remains the fame, is exactly as the' 

 force that comprefTes it. But the experiments referred to were 

 not accurate enough to eflablifh this law with abfolute preci- 

 fion ; and they left room to fufpect a deviation from it, either 

 when the compreffing force is very great or very fmall. Accord- 

 ingly, from experiments defcribed in the gib. vol. of the Mem. 

 of Berlin, it appears that the elaflicity of air of the temperas 



ture. 



* Tab. 2. and 3. p. 701. 703. Tranf. 1777. part 2. 



