ON HYDROSTATICS. Q^g 



^erties of fluids. la these departments, although we eftn by no means ex- 

 plain with precision the manner in which every appearance is produced, we 

 shall still find a variety of very beautiful phenomena, which have indeed been 

 too generally neglected, and supposed to be of the most abstruse and unin- 

 telligible nature; but which, when carefully examined, will appear to be 

 much more within the reach of calculation, than the simplest doctrines of 

 hydraulics. We may also apply some of these phenomena to a very complete 

 explanation of an extensive class of facts in optics, which, in whatever 

 other way they are considered, are inextricably obscure. Whether this ex- 

 planation may or may not be admitted as satisfactory, it deserves at least a 

 fair examination; it would, therefore, be impossible to assign to the science 

 of optics an earlier place in the order of the system, even if we agree with 

 those, who imagine that all the phenomena of light depend on causes wholly 

 deducible from the mechanics of solid bodies. 



We must commence the subject of hydfostaties, or the doctrine of the equili- 

 brium of liquids, With a definition of the essential characteristics of a fiui^i 

 substance. The most eligible definition appears to be, that a fluid is a 

 collection of material particles, which may be considered as infinitely small, 

 and as moving freely on each other in every direction, witliout friction. 

 Some have defined a fluid as a substance which communicates pressure equally 

 in all directions; but this appears to be a description of a property derivable 

 from the former assumption, which is certainly more simple; and although it 

 may be somewhat difficult to deduce it mathematically, in a manner strictly 

 demonstrative, yet we may obtain from mathematical considerations a suffi- 

 cient conviction of its truth, without assuming it as a fundamental or 

 axiomatic character. A fluid which has no immediate tendency to expand 

 when at liberty, is commonly considered as a liquid : thus water, oil, and 

 mercury, are liquids; air and steam are fluids, but not liquids. 



W6 shall for the present consider a liquid as without either compressibility 

 or expansibility: and we must neglect some other physical properties essen- 

 tial to liquids, such as cohesion and capillary attraction; although, in reality 

 the particles of liquids are found, by very nice experiments, to be subject to the 

 same laws of elasticity which we have already examined with regard to solids, 



