5 1 1 Rfintiritthlt Phentmetitn hi Metearohgy. 



Ever fince I made tliis obfervation, my thoughts have been frequently direfted to account 

 for if, and it was not till lately at Plombieres tlut I found a fitisfa£lory caufe, which I 

 fliall make the fubje£l of the prcfent Memoir. 



In order to give every polTibIc degree of certainty to rny obfervations, I kept my inftru- 

 ments defended not only from the dire£l rays of the fun, but from every rellcflion ; and I 

 obfcrved their (lation daily at the fame hours, but more particularly at four in the after- 

 noon, when the grealeft drynefs ufuaily prevails. 



During the two months that 1 remained at Plombieres, the greateft drynefs I obfervcd was 

 on the 2d of Augufl. The hygrometer indicated 68,5', the thermometer being at 22, 5». 

 T hrcc or four days before, the hygrometer at the fame hour had ilood higher or nearer to 

 humid, that is to fay, 86^, or 8;°, though the thermometer was near a degree higher, namely 

 23,1°, which muft have proportionally raifed the hygrometer. It rained in the evening of 

 the fame day on wliich it had been drieft, namely, on the 2d of Auguft. At the fame time 

 <hat the hygrometer dtfcended, the barometer alfo fell near two lines. Now I have proved, 

 in my Efl'ays on Hygronietry, and by fcveral experiments, that in an air which is rarefied the 

 hygrometer defcends, and denotes a greater degree of drynefs. I therefore attributed this 

 extr.-iordinary dryntfs to the rarcfaftion of the air. 



Cn my return from Plombieies, I purfued my obfervations wich tlie fame care. On the 

 igthof Augud of this prefent year, at 20 minutes after 4 in the evening, I obfervcd the hy- 

 grometer at 74°, while the thermometer ftood at 22,5°. On the following day, at 50 mi- 

 nutes after one, I found the thermometer exa£lly at the fame degree as the evening before, 

 namely, 22,5", the hygrometer being at 59,5', that is to fay, 14,5° higher than the evening 

 before. I noted this moment, as affording a valuable obfervation, on account of the iden- 

 tity of height in the thermometer. It was thc»efore evident that the drynefs of the air had 

 been increafed, not by its heat, but by fome otlier agent, fuch as its rarity. In fa£t, the ba- 

 rometer had defcended more than half aline. A wind from the fouth-weft prevailed at the 

 fame time, and it rained early the next morning. 



But the moft ftrlking inftance is to be found in my Eflay on Hygrometry. I made this 

 obfervation at Chamouni on the 23d of July 1781. The hygrometer ftood at 4 1,2^, the ther- 

 mometer indicating at the fame time 20,2°. But this degree of heat was far froni fuflicient 

 to produce fuch a degree of drynefs ; for, by calculating from the table at page 87 of my 

 Ell'ay on Hygrometry, we find that the difference of 4! degrees of heat between that day 

 and the preceding could not deprefs the hygrometer more than 9 degrees, inftcad of 2C, 

 •which it really defcended. The excefs, namely 1 1 degrees, mufl therefore be attributed to 

 another caufe, which I conclude to be the rarefadion of the air. The air may be rarefied 

 cither by the fall of the barometer, or the direiSlion of the wind. In fa£l, the fouth and 

 fouth-weft winds coming froVi lower countries than ours, and from the fea, muft neceflarily 

 rife and thence become rarefied, and confequently, as I have faid, caufe the hygrometer to 

 advance towards J»y. It is probable likewife that the fingular elevation of the valley of 

 Chamouni above the level of the fea, was one of the principal caufes of the drynefs which 

 predominated in that valley on the 26th of July 1781. 



We may therefore conceive the rcafon of this phenomenon, which at firft fight appears fo 

 ftrange ; namely, that extraordinary drynefs (liould precede rain, and that the hygrometer 

 fliould become, in tliis manner, an affiftant to the barometer, and afford one of tlie moft cer- 

 tain 



