of Barometrical Mensuration. 79 



people will sooner or later investigate this matter still farther, 

 not only by experiments made on the expansion of air taken at 

 different heights above the level of the sea in middle latitudes 

 but likewise on that appertaining to the humid and dry regions 

 of the atmosphere towards the equator and poles, that I have 

 been induced to hasten the communication of this paper. In 

 the mean time, having proved, beyond the possibility of doubt, 

 that a wonderful difference dotli exist between the elastic force 

 of dry and moist air, I may be allowed hereafter to reason 

 by analogy on the probable effects this will produce in mea- 

 suring heights by the barometer." Phil. Trans. ; vol. Ixvii. 

 p. 714. 



M. La Place, who has applied the prodigious powers of his 

 science to the perfecting the barometric formula, and has 

 availed himself of all the accuracy of the modern method of 

 experiment, was forced to leave the hygrometric state of the air 

 in the catalogue of inevitable errors, contenting himself with 

 an approximate correction. 



" Les corrections," says he, " relatives a la latitude, et a la 

 variation de la pesanteur, sont tres-petites ; mais commes elles 

 sont certaines, il est utile de les employer pour ne laisser sub- 

 sister dans le calul que les erreurs inevitables des observations, 

 et celles qui resultent des attractions inconnus des montagnes, 

 de I'etat hygrometrique de I'air, auquel il serait necessaire d^ avoir 

 egard et enfin de I'hypothese adoptee sur la loi de la diminution 

 de la chaleur. On tiendrait corapte en partie, de I'etat hygro- 

 metrique de I'air en augmentant un pen le coefficient 0.00375 de 

 y^' dans la formule precedente ; car la vapeur aqueuse est 

 plus leg^re que I'air, et I'accroissement du temperature en ac- 

 croit la quantite toutes choses egales d'ailleurs." — Mecanique 

 Celeste, Tom. iv. p. 292. 



The late lamented Mr. Playfair, in an elaborate paper upon 

 the same subject, published in the Philosophical Transactions of 

 Edinburgh, (Vol. i. 1778.) thus enforces the same argument; 

 " There is another cause of error which, had the effects of it 

 been sufficiently known, ought, no doubt, to have entered into 

 tliis investigation. Moisture when chemically united to air. 



