IV. 



FLUID FORCES. 

 I. A Pound Weight balancing a Hundredweight. 



WHEN those who have not studied the 

 physical sciences or had much experience of 

 practical mechanics are told that a hydraulic 

 press is a machine which will enable one man 

 to lift a weight -of fifteen tons, or that a large 

 one, worked by mechanical power, will lift 

 two thousand tons, it seems to them that 

 there must be some extremely clever, recon- 

 dite, and almost supernatural use made of 

 physical forces to produce so tremendously 

 large a power out of a small one. 



In reality, the result is obtained by a very 

 simple application of forces almost universally 

 present and active. 



All depends upon the fact that water, in 

 common with all other fluids, possesses the 

 property of transmitting pressure equally in 

 all directions. 



A piece of iron or stone, on the other hand, 

 is without this property. It is pressed by 

 gravity downwards, and can only transmit 

 the pressure downwards to things beneath. 

 Things at its side are not pressed by its weight. 



77 



