MECHANICS OF LIQUIDS 69 



rate of this change in shape varies with different liquids. 

 Those in which the change proceeds slowly are called viscous 

 liquids, while liquids in which the change takes place quickly 

 are called mobile. 



Another important property of liquids is that they cannot 

 be compressed. If force acts on any part of a liquid, it 

 will transmit the pressure of the force equally in all direc- 

 tions. This principle, which is called Pascal's law from its 

 discoverer, renders liquids very valuable as a medium for 

 pressure transmission in all forms of hydraulic machines. 



69. Water Pressure. Water exerts a pressure on the 

 bottom and sides of the vessel which holds it. Fill a vessel 

 1 cu. ft. in volume with water. If the water is weighed it 

 is found to weigh about 62.5 Ibs. Therefore 62.5 Ibs. is 

 pressing on the bottom of the box, the area of which is 144 



62.5 

 sq. in. Therefore the pressure per square inch is ~T7 or 



.434 Ib. The unit of pressure is the amount of pressure 

 to the square inch. Pressure equals force per unit area. 



A liquid also exerts pressure on the outside of any object 

 immersed or pushed into it and the pressure increases with 

 the depth. This phenomenon may be explained by consider- 

 ing a liquid as made up of a large number of thin horizontal 

 layers, each layer supporting the weight of those above. 

 The lower the layer, the greater the weight of liquid it has to 

 support; hence the greater the pressure exerted upon it. 

 This pressure has nothing to do with the size and shape of 

 the vessel and is evenly exerted upon each square inch of 

 surface. 



The total pressure of a liquid upon any portion of the 

 vertical sides of a vessel is equal to the weight of a column 



