ON THE SUPPOSED GMHIENTS IX HOT LIQUID** 17 



furface. By the time the liquid lias reached the temperature 



of the air, fome of the amber may be obferved floating on its 



furface, fomc of it a little below the furface, and, in fhort, in 



almoft every part of the liquid. After this the liquid may be 



left at reft as long as you pleafe j but the amber does not 



<hange its fituation, unleis fome very coniiderable change 



takes place in the temperature of the (unrounding air. Here 



we fee that the water cools down more rapidly than the 



amber ; fo that at a certain period the amber becomes fpeci- 



fically lighter than the water, and therefore riles ; extremely 



flowly indeed, becaufe the difference between the fpecific 



gravity of the two bodies is fmall. I fnppofed at firft that This afcent it 



the rife of the amber was owing to the defcent of the fuperior "u rrcn t. C y 



ftrata of the liquid, which, of courie, would have forced up 



the flrata in which the amber was ; but I was miftaken. Fox 



when the furface of the folntion was tinged yellow, the yellow 



ftratum remained neatly deiined, and without- changing its 



place during the whole of the proceis. 





Annotation. Atwood, in his Treatife on Motion, Sc&. V. Afcent of a ^ 

 Prop. 12, demonftratcs that a fphere of evanefcent weight fphcrcinafluIdj 

 will afcend in any denfe fluid uniformly with a velocity equal 

 to what it would have acquired by falling in free fpace 

 through four third parts of its diameter. This is the greatefl 

 poflible velocity of afcent of a fphere in any fluid. It may 

 feem abfurd to attempt a calculation, one of the aflumptiqns 

 of which fhall be the diameter of an integrant particle ; but if 

 we take any grofs quantity (fuch, for example, as the mil.- 

 lioneth part of an inch), for the diameter of a particle of water 

 at its utmoft expansion by heat, and deduce its velocity of 

 afcent (which in that cafe could fcarce amount to the two Motion of the 



hundredth part of an inch in a minute), we fhall find little reafon J*"** 01 " of watcr 

 - , by mere expan- 



to admit that afcending currents can be produced by the mere fian muft be ex- 

 enlargement of the particles heated only by contact with the tremel y flow « 

 containing veflel. T his, however, muft neceffarily be fuppofeo! 

 if the particles be npn-condu&ors with regard to each other. 



The excellent paper before us directs our attention to the Circulation of 

 jnechanical caufe of that circulation, which undoubtedly takes Ifffi,*^^** 

 place during the communication of heat to fluids from b«low, 

 3 and 



