148 THE WATERS OF THE EARTH 



small piston is one square inch and the area of the face 

 of the large piston is 100 square inches. If a pressure of 25 

 pounds were exerted downward on the small piston, an 

 equal pressure would be exerted upward on every square 

 inch of the face of the large piston. Thus 25 pounds pres- 

 sure on the small piston would cause an upward pressure of 

 2500 pounds on the large piston. 



In the operation of this press, the large piston would 

 rise only one hundredth as far as the small piston descended. 

 If the small piston descended a foot, the large piston would 

 rise one hundredth of a foot. In other words, the pressure 

 on either piston times the distance it travels equals the 

 pressure on the other piston multiplied by the distance it 

 travels. 



The enormous force that can be exerted by the hydraulic 

 press is used in baling cotton and paper, in punching holes 

 through steel plates, in extracting oil from seeds, in lifting 

 huge machines, and in many other devices where immense 

 pressure is needed. 



Buoyancy of Water. Experiment 68. Prepare a block of 

 wood having dimensions of 6x4x4 cm. Bore a hole in one 

 end of the block and fill it with sufficient lead so that it will 

 readily sink in water. Tightly close the hole containing the lead 

 and dip the block in melted paraffin to make it entirely waterproof. 

 Carefully measure the block and compute its volume in cubic 

 centimeters. 



Drive a small tack into the center of one of the smaller faces 

 of the block. Attach a thread to the tack and lower the block 

 into a cylinder graduated to cubic centimeters. Pour into the 

 cylinder more- than enough water to cover the block. Read on 

 the cylinder scale the combined volume of the block and the water. 

 Pull the block out of the water. Read on the scale the volume of 

 the water left in the cylinder. Does the difference between the 

 two readings equal the computed volume of the block? 



