280 Observations on the Hygrometer. 



tions, the elasticity of vapour was ascertained by means of Dr 

 Anderson's table, except in temperatures below zero, or greater 

 than 100°, when the elasticity was calculated, like the table, by 

 means of Biot's formula. Professor Leslie'*s formula was calcu- 

 lated according to the directions, and with the assistance of the 

 tables, contained in the article Meteorology, in the Supplement 

 to the Encyclopaedia Britannica. 



Correction for Pressure. 



In the preceding observations, B has been supposed = 30 

 inches. When the pressure exceeds or falls short of this, the 

 co-efficients of L and D require a correction. Regarding the 

 theory of this correction, however, and the exact amount of it, 

 there seems to be some difference of opinion. 



According to Professor Leslie, when the place of observation 

 is above the level of the sea, it must be increased by a quantity 



30 



proportioned to the altitude ; that is, proportional to log. — 



nearly. So that, on this theory, the same value of L indicates, 

 under a less pressure, a less -degree of dryness. 



Dr Anderson being of opinion that D is proportional to the 



30 

 quantity of water evaporated, that is, generally, to — (ft-—f'\ 



proposes, as the correction for pressure, to multiply D by — -' 



and consequently he conceives the same value of D to indicate, 

 under a less pressure, a greater degree of dryness. Dr Ander- 

 son's formula, corrected for pressure, is 



Mr Meikle appears to agree with Dr Anderson as to the na- 

 ture, though not as to the amount, of the correction. Mr 

 Meikle's experiments seemed to shew that D is nearly propor- 

 tional to ^ ^'^-- , It does not appear, however, whether Mr 

 B + 27 



Meikle considers ^' = < ~ D, or ^ = t —- D ; that is, 



whether the value of D only is to be corrected, or whether both 

 D and/ are to ho. corrected for pressure. 



It will be observed that it is the object, both of Dr Ander- 

 son s and Mr Meikle's experiments, to ascertain how D varies 



