and Levelling, fyc. 320 



Introducing the effect of aqueous vapour upon the density 

 of dry air 



»-»(*-!£) » 



where /is the elastic force of aqueous vapour, and B the height 

 of the barometer. 



In like manner, if R' be the refringent power of the air 

 under the same circumstances, while R is that at the pressure 

 B, the standard at which most of our experiments have been 

 made, that is, about m .76 of the metrical barometer, or 

 29.9218 inches of the English barometer, 



E' = R( r i + ^-) . — L_ .... (3) 



V 12 B/ 1 _ 3/ 



8B 



Now if A be the density of mercury at the freezing point 

 when B is the height of the barometer, 



1= ^- B = 104GG.8B = 26100 English feet .... (4) 



when B = m .76, or 29.9218 inches, the pressure at which 

 ^= 10466.8 was determined. 



The expansion of dry air for 1° of the centigrade thermo- 

 meter here designated by /3 is generally estimated at 0.00375 ; 

 hence 



B' = B (1 + /3 (5) 



therefore by substitution for B and D in formula (4), 



A 1 +fit _ 1 +/3< 



5 — u~ rjj 



8B 8B 



Putting these in equation (1) and n = w D' -^ becomes 



r / / \ 8 B 



The value of /, the elastic force of aqueous vapour, may be 

 taken from the well-known table of Dalton, or from one given 

 in my Mathematical Tables from a formula of Mr Ivory, 

 founded on the experiments of Dr lire. If thought necessary, 

 in such cases as this the true elasticities may be determined 



