18 PHYSICS. 
the pressure on D, and through D on the articles between the two plates 
of the press, ould be 500,000 pounds, or above 200 tons. 
The pressure caused by a piston being forced into a vessel filled with 
water, may also be caused by the weight of water itself; for whether the © 
piston at a (fig. 18) be forced in with a pressure of one pound, or one pound- 
weight of water stand in the tube, the pressure in both cases is the 
same. In this way a very strong cask may be burst by a few ounces of 
very narrow tube inserted in the top of the cask. Ifthe tube 
hold only half a pound of water, and the bore of the tube be 
one-fortieth of a square inch, the pressure of the water in the 
tube will cause a pressure, transmitted by the water in the 
cask, of half a pound on every one-fortieth of an inch of the 
inner surface of the cask—that is, of nearly 3000 pounds on 
every square foot—a pressure which uo ordinary cask could 
bear. 
of the simple means by: which the operations of nature are 
rock by successive falls of rain will ultimately rise to such a 
height as to cause a pressure sufficient to burst asunder from 
the mass a large portion of the rock. 
2. In a liquid mass, there is a pressure increasing in intensity with the 
perpendicular depth. 'The truth of this will be at once clear if we suppose 
a mass of liquid divided into thin horizontal layers. The upper layer 
must be supported on the second, and the pressure on the second layer 
is the weight of the first. And, since the second layer must be supported 
by the third, the pressure on the third layer is the weight of the two 
above it; and so on to any depth, the pressure at any depth always being 
the weight of the water above, so that it must always be proportional to 
the perpendicular depth. And this is the case whatever may be the shape 
or width of the vessel. Every one must have noticed with how much 
greater force water rushes from a deep vessel when the opening is near 
the bottom, than when it is near the top, or when the vessel is nearly 
empty: this difference is caused by the difference in the pressure of the 
water above. . 
3. The free surface of a liquid mass in equilibrium is. a perfect level. 
Since the particles of a liquid are freely movable among one Another, it 
follows that if the liquid were heaped up at any particular part it would 
always slide down again till the surface was level. There is a case, how- 
_ ever, in which the fact is not perfectly clear at first sight—namely, when 
the vessel consists of different parts communicating with each other. 
A common tea-pot will afford a convenient illustration. The water stands 

water. In fig. 20, a is a cask filled with water, and Db is a 
This bursting of the cask is an illustration, on a small scale, 
effected. For example, the water poured into a crevice in a - 
a a i i 
