38 SCIENCE PRIMERS. [MATERIAL 



pressure will become greater and greater until the 

 hand is driven away, or the cork is forced out, and the 

 water falls to the ground. The pressure in this case 

 is the same as the weight of the water, and the cork 

 would have been driven out equally well by a rod of 

 lead of the same weight. 



Suppose the tube to be square, and that the inside 

 of the square measures exactly one inch each way. 

 Then an inch of height of the tube will hold exactly 

 one cubic inch of water. Since one cuDic inch of 

 water weighs 252 grains and a half, as much water 

 as will fill the tube about two feet three- inches 

 and a half high, will weigh a pound (7,000 grains), 

 and fifteen pounds of water will fill such a tube 

 between thirty-three and thirty-four feet high. And 

 these respective weights measure the pressure of two 

 columns of water, one twenty-seven and a half inches 

 high, and the other nearly thirty-four feet high, on a 

 square inch of the surface on which they rest. 



The specific gravity ( 24) of lead is n'45; in 

 other words it is about eleven and a half times denser 

 than water. Therefore if a bar of lead cut square 

 and one inch in the side, and rather less than ^rth 

 of the height of a column of water, is slipped into 

 the tube in place of the water, it will exert the same 

 pressure on the bottom. 



And now comes a difference between the lead 

 and the water, which depends on the fluidity of the 

 latter. The lead exerts no pressure on the sides of 

 the tube, but the water does. If a small hole is cut 

 in the side of the tube close to the bottom, and 

 stopped with a cork, the lead will not press upon 

 the cork. But if the column of water is high enough, 



