352 Hydrostatics. 



instead of entering a basin the tube turn up at the bottom, as in 

 figure 230, so as to admit the air at C, the perpendicular elevation 

 above a horizontal line coinciding with the surface at E or F, 

 will be the measure of the atmospheric pressure. This eleva- 

 tion, moreover, is independent of the form of the tube and the 

 particular quantity of mercury contained in it. On the suppo- 



l ^' sition, however, that the base of the tube is an inch square, the 

 pressure is equal to that of a parallelepiped of mercury 30 in- 

 c' es in length, or, which amounts to the same thing, to the 



410. weight of 30 cubic inches of mercury. Now 30 cubic inches of 

 water is equal to 30 X 252, 525 grains, or 15,78 troy ounces. 

 Whence 30 cubic inches of mercury is equal to 15,78 X 13,57 

 or 214,12 troy ounces; that is, to 234,7 ounces avoirdupois, or 

 to 14,7 lb * We infer, therefore, that the pressure of the atmos* 

 phere amounts to nearly 15 lbt upon every square inch of surface, 

 or to about one ton upon every square foot. A common sized 

 man exposes a surface of 10 or * 1 feet, and is consequently sub- 

 jected to a pressure of as many tons' weight. The entire surface 

 of the earth being estimated at 5575680000000000 feet, this 

 number will express the weight nearly of the whole atmosphere 

 in tons, a certain deduction being made for the space occupied 

 by mountains and elevated regions. This pressure being exert- 

 ed upon the surface of the ocean, fishes are exposed to it in 

 addition to the weight of their natural element. But the pro- 

 portion 



1 : 13,57 :: 30 : 407,1, 



gives 407,1 inches, or 34 feet nearly, for the length of a column 

 of water equivalent to that of 30 inches of mercury or the pres- 

 sure of the atmosphere. Accordingly for every 34 feet depth 

 a pressure is exerted of a ton upon every foot of surface, over 

 and above that arising from the atmosphere. Now fishes are 

 sometimes caught at the depth of 2600 or 2700 feet, where the 

 pressure of the water amounts to nearly 80 atmospheres or 80 

 tons upon a square foot ; yet these fishes are not injured by such 

 an immense weight, or sensibly impeded in their motions. The 

 reason is, that they are filled with fluids, which from their im- 

 penetrability oppose a sufficient resistance to this pressure, and 

 thus preserve the most delicate membranes from being ruptured. 



