4 1 8 dBian of Fluids on folid Bodies. [Book VII. 



fiphon continues to flow without intermiffion ; becaufe 

 the refiftance of the air in C is as much exceeded, as 

 the length of the branch B C of the fiphon exceeds 

 that of the branch A B. In order to prove this, 

 fuppofe there is added at C a tube to lengthen that 

 branch, then it will plainly appear, that in a given 

 time more water will flow than would have been dif- 

 charged without that augmentation to the branch BC. 



Since it is the preffure of the air which elevates the 

 fluid in the fhort branch B A, it follows, that the 

 height of this branch is limited to thirty-two feet 

 when the fluid is water, becaufe the preffure of the 

 atmofphere cannot elevate water higher ; but when 

 the fluid is mercury, the height of the (hort branch 

 fhould not exceed thirty inches, becaufe the atmofphere 

 cannot fuftain mercury at a greater height. 



III. The action of fluids on folid bodies immerfed 

 in them, has been already in part anticipated, in treat- 

 ing of fpecific gravity. It is neceffary, however, to 

 refume that fubje<5t to a certain extent in this place, 

 and I fhall endeavour as much as poflible to avoid re- 

 petition. 



It is evident, that when a folid body is plunged into 

 a fluid, it occupies a fpace in that fluid exactly equal 

 to its own magnitude. The quantity of fluid then fo 

 difplaced, either equals in denfity and confequently in 

 weight, the folid which difplaced it ; or, on the con- 

 trary, one of the two mult weigh more than the other. 

 In the lad cafe, which is moft common, the quantity 

 by which the heavier body furpaffes the lighter, is 

 called the fpecific weight or gravity. 



It has been obferved before, that fluids exercife 



their preffure in all directions, confequently a folid 



body plunged into a fluid is preffed at all points. IE 



has alfo been proved, that this preffure increafes in 



2 proportion 



