"Chap, .] Particles of Fluids. 403 



are moveable among themfeives, having fcarcely any 

 cohefion one with another, and moving independently 

 of each other. Some philofophers * have included 

 in this definition what they term the grofier fluids, as, 

 for example, a heap of corn, a heap of fhot, of fand, &c. 

 as well as the rarer and more elaftic fluids, as common 

 air, and all other aeriform fubftances. The proper 

 objects, however, of the hydroftatic icience, are thofe 

 fluids which, in common language, are termed liquids, 

 or thofe which always prefent to us a plane furface, 

 level or parallel to the horizon. 



All liquids are not equally fluid ; hence it fol- 

 lows, that the laws of hydroftatics apply with lefs ex- 

 actnefs in proportion as thofe fubftances depart from, 

 perfect fluidity. Water and oil both flow when the 

 vefiels, which contain them, are either overturned or 

 broken j but the effufion of oil is flower than that of 

 water, becaufe the particles of oil have more coheflon 

 among themfeives. The moft fingular effects in hy- 

 droftatics principally depend upon the extreme mi- 

 nutenefs of the particles of fluids, and upon their great 

 mobility. 



To preferve a lucid order in the confideration of 

 this fubject, it will be necefiary to divide the objects 

 of our inquiry into three branches. In the firft place, 

 therefore, I fhall confider in what manner the principle 

 of gravity acts on the particles of fluids, and the phe- 

 nomena which it produces in the fluids themlelves ; 

 as well as their action againft the fides, the bottoms, 

 and tops of the veflels in which they are contained. 

 Secondly, I fhall confider in what manner fluids of dif- 

 ferent denfities act upon each other -, and thirdly, the 

 aftion of fluids on bodies immerfed in them. 



* See Briffon, Vol. I.j>. 233. 



D d i I. In 



