PHYSICAL AND LITERARY. 8i 



fent fliort efTay, is to tuke a view of the 

 caufes affined, and to examine whether 

 any of them can be relied on as the true 

 caufe. 



Tk e firft I {hall mention is that given 

 by Dodor Halley. He imagines that a 

 bubble, compofed of a particle of air inclo- 

 fed within a film of water, is rarined by 

 heat' to the degree of being fpecifically 

 lighter than common air ; and that fnch 

 particles, whatever be their number, muft 

 afcend and be fufpended in the air. This 

 theory is liable to many objedlions ; fomei 

 of which appear unfurmountable. A 

 great quantity of air, it is true, is found 

 difperfed in water ; but by what means 

 a bubble Ihould be formed diftincl ti oai 

 the mafs of w*ater, feems not obvious ; 

 and ought not to be taken for granted, 

 till it be afcertained by good proof. But, 

 fuppoiing the exiilence of J'uch bubbles, 

 they will not advance the Doctor a fingle 

 flep ; for it flill remains to be explained 

 by what means thefe bubbles afcend in- 

 to the air. Gravity is evidently an in- 

 fuiEcient caufe j for, if it cannot force 

 Vol in, L upward 



