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The infufFiciency of the fecond reafon alledged by Dodlor 

 Halley has been clearly fliewn by Mr. De Luc ; nor is the expe- 

 riment of Mr. Hawkfby conclufive, as it appears that part of the 

 air already confined in the boxes was forced out by the blaft of 

 air ; and to remove all doubt of the infufficiency of this expla- 

 nation, I need only mention the obfervation of Mr. Derham, 

 that during the greateft vehemence of ftorm the $ rifes inftead 

 of falling lower. IV. Phil. Tranf. Abridg. Part II. p. 77. And 

 I have had occafion to make the fame obfervation on the 28th 

 of February, 1785, in London. 



Of the Influence of Fapors. 



The influence of vapors was never totally overlooked by any 

 of the philofophers who undertook to explain the variations of 

 the barometer, though the part they really ad was little under- 

 llood by any except Mr. De Luc. However, within thefe few 

 years the greateft light has been thrown on this fubjed by Mr. 

 De Sauflure, in his incomparable treatife on hygrometry. He 

 very juftly diftinguiflics two forts of vapors ; the irivi/tble, to which 

 the name of vapor is moft properly applicable, and the vijible ; of 

 this Idft there are two forts, the vejicular and the concrete. The 

 invifible are fpecifically lighter than air of the fame temperature, as 

 Mr. De Luc has fliewn by numerous obfervations, and Mr. De 

 Sauflure by dired experiments ; but the vejicular are of the fame 

 fpecific gravity as the air in which they fubfift. To underftand 

 the influence of both fpecies of vapors on the barometer, it is 

 neceffary to enumerate fome of their principal properties, and 

 fliew how air is aifeded by them. 



First. 



