[ 278 ] 



materially benefited this fclence by regiftring the variations of 

 their barometers and thermometers during their voyages. 



Of the Moi/lure of the Atmofphere, 



Moisture may be confidered as rifing into the atmofphere, or 

 fubfifting in it, or defcending from it. I fhall firft confider its 

 afcent. 



CHAP. I. 



Of Evaporation. 



Evaporation Is the converfion of a liquid (and frequently 

 even of a folid) into an invifible fluid. That the liquid is di mi- 

 nified is evident from its lofs of weight ; but that the aqueous 

 liquid, which 1 have here principally in view, is not decompofed, 

 is equally clear, fince the invifible fluid, into which it is converted, 

 may be made to re-appear in its former vifible ftate. 



This converfion may be efFeded either naturally or artificially : 

 that produced by nature is always accompanied by contad with 

 the atmofphere ; the artificial is producible in contad with the at- 

 mofphere, or even in vacuo, as is now well known. 



In the common courfe of nature five caufes concur in produ- 

 cing evaporation : heat, afiinity to atmofpheric air, agitation, elec- 

 tricity, and light. 



Section 



