of Barometrical Mensuration. 77 



and Laplace, have been applied to the latter. But one desi- 

 deratum in physics has stopped the progress of each at nearly 

 the same point; a desideratum which all have felt, and all in 

 succession have pointed out. I allude to the deficiency of 

 means to measure the quantity and effects of aqueous vapour 

 in the atmosphere. The relation of the air's density and elas- 

 ticity, the effects of heat upon the relative weights of mercury 

 and air, the diminution of gravity in ascending from the surface 

 of the earth, its variation in different latitudes, and the dis- 

 turbance of centrifugal force, have been appreciated and allowed 

 for ; but all the corrections, except the two first, are exceeded 

 in value by that which has hitherto been only the subject of 

 conjecture, namely, the correction for moisture. Some of the 

 latter calculations have indeed assumed an appearance of con- 

 siderable accuracy, but while the more important problem 

 remains unsolved, such appearance is merely illusory, and it 

 may fairly be doubted, whether the state of physical science is 

 ever likely to be such, as to render the introduction of the re- 

 finements which they exhibit, practically advantageous. The 

 importance, however, of the problem, the solution of which I 

 am now about to attempt, has on the contrary been universally 

 admitted. 



M. De Luc, in his valuable and laborious " Researches upon 

 the Modifications of the Atmosphere," thus adverts to the 

 knowledge which it is necessary to obtain of the effects of 

 vapour in the air for the perfecting the mensuration of heights 

 by means of the barometer. 



" Voilk done un nouveau champ ouvert aux experiences. II 

 s'agit de determiner quel changement on doit faire ^ la hauteu 

 trouvee par les logarithmes, quand I'air est plus ou moins 

 charge de vapeurs qu'un certain point fixe, et de vapours 

 echauffees plus ou moins qu'un certain degre. II me semble 

 que, pour decouvrir cette loi, 11 faudroit pouvoir Joindre 

 I'observation d'un hygrom^tre comparable k celle du barom^tre 

 et thermometre. Car le point essentiel consiste k connoitre 

 s'il y a des vapeurs dans la colonne d'air qui est interceptee par 

 les deux stations, et quelle est Icur quantite, puisquc, si les 



