128 CAUSES -which affeEl the ACCURACY 



mogeneous and invifible fluid, appears to have a powerful effect 

 to encreafe the elaflicity of the air, and its expanfion for every 

 additional degree of heat which it receives. In experiments 

 -with the manometer*, it has been obferved, that, till the 

 moifture was diffolved in the air, it had no fenfible effect: on its 

 elaflicity j but that, as foon as it began to diffolve, the expan- 

 fion, for one degree of heat, was encreafed, and continued to 

 be fo, for every fucceflive addition of heat, from thence to the 

 boiling point, where it became nine times- that of dry air. 

 From this, too, it probably proceeded, that, at Spitzbergen, 

 within ten degrees of the pole, a place where the circle of per- 

 petual congelation in the atmofphere, approaches near to the 

 furface of the earth, and where the air may naturally be fup- 

 pofed to be very dry, the ufual rule for the meafurement of 

 heights was found to err greatly in excefs, and it appeared, that 

 the denfity of the air was greater than could have been inferred 

 from its compreffion and its temperature. 



43. THOUGH the judicious and accurate experiments of Ge- 

 neral ROY have afcertained this effect of humidity, and have 

 even gone far to determine the law of its operation, yet, for 

 want of a meafure of the quantity of it, contained, at any 

 given time, in the air, it is impoflible to make any applica- 

 tion of this knowledge to the object under our confideration. 

 While I was reflecting on this difficulty, it occurred, that the 

 barometer itfelf might become a meafure of the humidity 

 of the air, and that the error committed in the meafuring of a 

 known height, if all other circumftances were taken in, would 

 determine the quantity of that humidity. For, if we fuppofe, 



that the formula z =. p(i+m( r^log. gives the true 



height between the flations at* which two barometers have 

 been obferved, when the moifture diflblved in the air is of 

 its medium quantity, (which we may call unity), then, if that 



moifture 



* See General ROY'S experiments, fedtion 2. Phil. Tranf. vol. 67. part 2. 



