Jome heated Bodies in cooling, i*]^ 



gradually acquired, as it cooled, an increafe 

 of weight, lb that, at the end of twenty-two 

 Jiours, it weighed fix penny weights, feventeen 

 grains more, than it did when firfl: committed 

 to the balance. This phsenomenon, which, 

 by fome has been adduced to prove, that heat 

 is the principle of levity in bodies, Mr. White- 

 hurft has endeavoured to explain, by fuppo- 

 fing, that the air, above the fcale being rare- 

 fied by the heated iron, the cold air below 

 rulhed up, and, flriking againft the bottom of 

 the fcale, not only prevented its defcent, but 

 even buoyed it up. Something may perhaps 

 be attributed to this caufe. But would not the 

 circumambient air beneath the fcale be nearly 

 as much rarefied as that above ? And is it not 

 probable, that the fuppofed force of this cur- 

 rent of air, would be, in great meafure, coun- 

 terafted by the greater tendency a body has 

 to defcend in a rarefied, than in a denfe medium ? 

 Is it not probable, likewife, that the end of the 

 beam, to which the heated iron was appended, 

 would by the fame heat which rarefied the air, 

 be more expanded, and lengthened, owing to 

 its nearer approximation to the fource from 

 which the heat flowed, than the more diftant 

 end of the beam ? I would likewife obferve, 

 that in the experiment of M. Buffon above 

 quoted, and in one made by Dr, Roebuck on a 

 fmaller fcale, the mafs, owing perhaps to the 



joint 



