410 . Fluids prefs m proportion [Book VII. 



for it muft neceftarily be of the fame height there as in 

 all the reft. 



From what has been obferved, viz. that when all 

 the parts of the fame fluid are in equilibrium, their 

 furfaces will alfo be in a plane parallel to the horizon, 

 or, in other words, every part of the iiirface at an 

 equal diftance from the center of the earth, it follows, 

 that when the furface of water is very large, it becomes 

 neceflarily and fenfibly convex. This is eafily per* 

 ceived at fea, where the mafts of fhips are cbferved at 

 diftance before any other part of the Ihip can be dif- 

 tinguiihed. 



4thly, Fluids prefs as well perpendicularly as late- 

 rally, not, however, in proportion to their quantity, 

 but in proportion to their height above the plane of 

 the horizon, 



For example, if feveral veflels of the fame height 

 and bafe are filled with water, all their bottoms will 

 bear the fame degree of preflure, whatever may be 

 the form and fize of the veflels in other refpec~ts. 

 Suppofe three veflels to be filled with water, A B C D 

 (fig. 4.) E F G H (fig. 5.) L M N O P QL(fig. 6.) 

 \vhofe heights A B, I F, L T are equal, and alfo 

 fupported by equal bafes B C, F G, N O j it will be 

 found, by experiment, that all the bottoms of thefe 

 veflels will be equally prefled, though the quantities 

 of water which they contain may be very different. 

 In the veffel (fig, 4.) the bottom B C is prefled by 

 the whole mafs of water A B C D, becaufe the fluid, 

 in this cafe, prefles in the fame manner as a folid 

 body ; let us fuppofe the weight of the water to be 

 fix- pounds: in the vefiel (fig. 5.) it is eafy to con- 

 ceive, from what has been faid before, that the bot- 

 tom F G is alfo only prefled by fix pounds, though 



the 



