NOTES. 



NOTE A. CHAPTER I. 



ARTICLE 3. Every particle of a non-elastic fluid presses equally in every 

 direction. 



The truth of the principle enunciated in this proposition, is abundantly illus- 

 trated by the experiments introduced at the end of the sixth chapter, and conse- 

 quently, it needs no further confirmation here ; but from it we may infer, that 



The lateral pressure of a fluid is equal to its perpendicular pressure, 



Art. 4. Every particle of fluid in a state of quiescence, is pressed equally in all 

 directions. 



This is obvious ; for if possible, let any particle receive a greater pressure in one 

 direction than in another ; then, since by art. 2, the particles of a fluid yield to the 

 smallest force or pressure, and are easily moveable amongst themselves, it follows, 

 that motion will take place in that direction in which the pressure is greatest ; but 

 by the proposition, the fluid is in a state of quiescence ; that is, there is no motion 

 taking place among its particles ; they are therefore equally pressed in all direc- 

 tions. 



Art. 5. When a fluid is in a state of rest, the pressure exerted against the 

 surface of the vessel which contains it, is perpendicular to that surface. 



This also is manifest ; for if the pressure be not perpendicular to the containing 

 surface, the re-action of that surface cannot destroy it ; let the pressure therefore 

 be resolved into two, the one perpendicular and the other parallel to the surface ; 

 then it is manifest, that the former will be destroyed by the re-action, and the 

 latter continuing to act on the particles of the fluid, will be transmitted in every 

 direction, and consequently, motion will take place ; but this is contrary to the 

 supposition, for the fluid is stated to be at rest ; therefore, the pressure must be 

 perpendicular to the surface. 



Art. 6. When a mass of fluid is in a state of rest, its 

 surface is horizontal, or perpendicular to the direction of 

 gravity. 



For let A B D c represent a vessel of fluid, such as water, 

 and conceive the right line A B to be parallel to the hori- 

 zon. Suppose the surface of the fluid to be in the position 

 PB, any how inclined to the horizontal line AB; then, 

 since by art. 2, the particles of the fluid are easily 

 moveable among themselves, it follows, that the higher 

 VOL. i. 2 G 



