Of BAROMETRICAL MEASUREMENTS. 95 



alfo reckoned too great. The effect of both thefe errors is of the 

 fame kind, tending to make the height lefs than it is in reality ; 

 yet it is only the firft of them, and that too the leaft confide- 

 rable, which has hitherto been taken into account. 



12. IT were to be wifhed,that, to the caufes here enumerated, 

 and that are to be introduced into the computation, we could 

 add the operation of moifture, in altering the weight and ela- 

 fticity of the air. But the law of that operation has not yet 

 been difcovered ; and it will be fufficient to point out, in the 

 conclufion of this paper, a method by which it may be deter- 

 mined from obfervations of the barometer itfelf. 



BEFORE proceeding to the inveftigation of the effect which 

 all thefe inequalities together muft produce, it is proper to re- 

 mark, that the two inequalities in the expanfion of air, taken 

 notice of ( 5. and 7.), after having been difcovered by Ge- 

 neral ROY, were applied by him to correct the height of moun- 

 tains, meafured by the barometer ; but that it is, by no means, 

 certain, that he has given to thofe corrections the precife form 

 which they ought to have. This, indeed, cannot be known, un- 

 lefs the effect of each inequality, on a fingle ftratum, be firft in- 

 troduced into the differential equation between the denfity of 

 the air and the height above the furface, and the amount of its 

 effect on a whole column of air be deduced from thence by in- 

 tegration. 



13. LET j, then, be the denfity of the air, at any height x 

 above the furface of the earth, the heat at the furface be- 

 ing H y expreffed in degrees of FAHRENHEIT'S thermometer. 

 If alfo x be fuch a number, that x# gives the degrees by which the 

 thermometer ftands lower at the height x than at the furface 

 (4.), the temperature at the height x will be=H *x; and, 

 if the expanfion of a given quantity of air, which occupies 

 the fpace i, and is of the temperature 32, for i of heat, be 

 called OT, then, abftracting at prefent from that inequality of 



expanfion 



