CHAMBERS'S INFORMATION FOR THE PEOPLE. 



made to balance one hundred pounds on the short 

 arm. When we think of the machine in motion, 

 we see that what is gained in power is lost in time. 

 If the piston in a descend one inch, it will raise 

 the piston c only the one-thousandth part of an 

 inch ; for a descent of one inch in a dislodges a 

 cubic inch of water, forcing it into the box ; this 

 causes a cubic inch of water to rise into c, where 

 it is spread out over a thousand times as much 

 surface, and therefore has only the thousandth part 

 of the depth. 



If the pressure we have supposed exerted on 

 the piston a arose from a pound of water poured 

 into the tube above it, it would continue the same 

 though the piston were removed. The pound of 

 watec in the tube is then pressing with its whole 

 weight on every square inch of the inner surface 

 of the box downwards, sidewise, and upwards 

 and, if the box is twenty inches each way, so as to 

 have upwards of 2000 inches of surface, this one 

 pound of water is tending to burst it with a force 

 of about half a ton. That this is no mere theory, 

 may be proved without much difficulty, by fitting 

 a long small tube, b, into the top of a cask, a, as 

 represented in fig. 2. If the tube is only the 

 twentieth of an inch in area, and is made long 

 enough to contain a pound of water, it gives a 

 bursting force of twenty pounds on every square 



Fig. 2. 



Fig- 3- 



inch of the inner surface a strain which no ordin- 

 ary cask could resist 



The apparatus called the hydrostatic bellows acts 

 on the same principle (see fig. 3). It consists of 

 two stout circular boards connected together by 

 leather in the manner of a bellows, B. The tube 

 A is connected with the interior ; and a person 

 standing on the upper board, and pouring water 

 into the tube, may lift himself up. If the area of 

 the upper board is loco times that of the tube, an 

 ounce of water in the tube will support 1000 

 ounces at W. 



It is on the principle explained with regard to 

 the two pistons, a and c, in fig. I, that the very 

 useful machine called the Hydraulic Press, in- 

 vented by Bramah, is constructed. Fig. 4 repre- 

 sents the essential parts of the machine, the details 

 of construction being omitted. F is the cavity of 

 a strong metal cylinder, E, into which the piston 

 D passes water-tight through the top. A tube, G, 

 leads from the cylinder to a force-pump, H ; 

 and by means of this, water is driven from the 



226 



tank T into the cavity F, so as to force the piston 

 D upwards. The 



piston supports a A % A 



table on which are 

 placed the bales, 

 books, or other 

 articles to be 

 pressed ; and the 

 rising of the table 

 presses them 

 against the entab- 

 lature AA, which is 

 fastened to the pil- 

 lars B, B. 



The power of the 

 hydraulic press is 

 readily calculated. 

 Suppose that the 

 pump has only one- 

 thousandth of the area of F, and that, by means 

 j of its lever handle, the piston of the pump 

 ' is pressed down with a force of 500 pounds, 

 the piston of the barrel will rise with a force of 

 i one thousand times 500 pounds, or more than 200 

 ! tons. The rise, however, will be slow in pro- 

 portion to the gain of power. 



PRESSURE OF WATER ON SURFACES IN VARIOUS 

 POSITIONS. 



Water has weight like solids, and this weight 

 produces pressure ; but the quality by which it 

 transmits pressure in all directions, makes its 

 weight be felt in many respects differently from 

 that of solids. Thus, suppose a cylindrical vessel 

 filled with a piece of ice exactly fitting it ; the 

 whole weight of the ice will rest on the bottom, 

 the sides being unaffected. But if the ice is now 

 melted, the bottom will sustain the same weight 

 as before, and the sides will at the same time 

 be pressed outwards with a certain force. Any 

 object immersed in water is also pressed on all 

 parts of its surface in contact with the water. We 

 have now to determine the amount of this pres- 

 sure caused by the weight of water in various 

 circumstances. 



The pressure of water increases in intensity with 

 the depth, -without regard to 

 the shape or size of the cavity 

 or -vessel containing it. 



Suppose the water in the two 

 vessels represented in fig. 5 to be 

 divided into layers of an inch 

 deep. I f the area of the tube C is 

 one square inch, and that of the 



, larger cylinder, A, ten inches, ' 



the top layer in the tube will 

 contain a cubic inch of water, 

 and that in the cylinder ten 

 cubic inches. Now, thes.e 



I layers rest on the surfaces of 

 the layers below them ; the 



j second layer in the tube sus- 

 tains the weight of a cubic inch Fig. 5. 



; of water ; that in the cylinder, 



i ten cubic inches. But in the last case the 

 weight is equally distributed over ten square 

 inches of surface ; in both cases, then, the pressure 

 on a square inch of the upper surface of the 

 second layer is the weight of a cubic inch of 

 water, or about 252 grains. In like manner, each 



