HYDROSTATICS. 1 13 



(that is, lighter than an equal bulk of the fluid,) 

 will ascend to the surface, if immersed below it. 

 For when such a solid is immersed in a fluid, it 

 presses downwards with a force equal to the weight 

 of a column composed of the body itself, and that 

 portion of the fluid which lies upon it; and the 

 fluid presses upwards against the body with a 

 force equal to a column of itself alone : hence 

 the body will rise with a force equal to the 

 difference between the pressures of these two 

 columns. 



In the same manner, a body specifically heavier 

 than a fluid must sink in it, since it must press 

 downwards more than the fluid presses against it 

 upwards. 



Thus we see the reason why a piece of cork is 

 more buoyant than a piece of oak, and also why it 

 is easier to swim in salt than fresh water. 



If by any contrivance, the force with which a 

 body presses downwards, and that with which the 

 fluid presses upwards, can be reduced to an 

 equality, then the body will remain suspended in 

 the fluid. Thus a piece of lead may be made to 

 swim in water, by immersing it to a proper depth, 

 and keeping the water from getting above it. Let 

 C D (Plate 8. fig. 7«)» De a glass tube open through- 

 out ; and, G, a flat piece of lead, exactly fitted to 

 the lower end of the tube, but not to go within it, 

 with a wet leather between the lead and the tube, 

 to make it air-tight. Let this leaden bottom be 

 half an inch thick, and held close to the tube, by 

 pulling the packthread, L, upward with one hand, 

 whilst the tube is held in the other by the upper 

 end. In this situation, let the tube be immersed 

 in water, in the glass vessel, A B, to the depth of six 



VOL. I. i 



