2(53 tECTUBE XXT. 



or nearly 1000 ounces; if we have a vessel of mercury an inch in depth, each 

 square foot will sustain a pressure of one twelfth part of a cubic foot of mer- 

 cury, or 11 30 ounces; the atmosphere presses on each square foot of the earth's 

 surface with a force of about 34000 ounces, which is equivalent to the pressure 

 of a column of mercury 30 inches high. The pressure of the water on a small 

 portion of the lowest part of the side of the vessel containing it, is also equal 

 to the weight supported by an equal portion of the bottom; but we cannot esti- 

 mate the force sustained by any large portion of the side, without considering 

 the difl'erent depths below the surface, at which its difierent parts are si- 

 tuated. 



It is obvious that if wc conceive a fluid to be divided by an imaginary 

 sorface of any kind, the particles contiguous to it are urged on either side by 

 equal forces, the fluid below resisting them, and pressing them upwards, 

 with as much force as the fluid above presses them downwards, their own 

 weight being comparatively inconsiderable, for without this equality of 

 pressures, they could not possibly remain at rest. And if we employ a 

 vessel of such a form as to occupy the place of any superior portion of the 

 fluid, the pressure against that part of the vessel which is thus substituted 

 will be the same that before supported the weight of the fluid removed; and 

 in order that all may remain in equilibrium, the vessel must itself exert an 

 equal pressure on the fluid below it; so that the pressure on the bottom will 

 be the same as if the vessel had remained in its original state, and were filled 

 to the same height with^the fluid. (Plate XIX. Fig. 242.) 



In order to understand this the more readily, we may suppose the portion 

 of the fluid, instead of being removed, to have been congealed into a solid 

 mass of equal density; it is obvious that this congelation of the fluid would 

 not have altered the quantity of its pressure ; it would, therefore, have re- 

 mained in equilibrium with the water below; the mass might also be united 

 with the sides of the vessel, so as to form a part of it, without increasing or 

 diminishing any of the pressures concerned : and we should thus obtain a 

 vessel similar to that which was the subject of our investigation, the pres- 

 sure on the bottom being always the same, as if the mass, supposed to be 

 congealed, had remained fluid. Thus, the pressure on the base of a conical 



