346 



HYDROSTATICS 



Pressure on the Upper Surface of a Liquid. If the surface 

 of a liquid is subjected to an external pressure, this pressure is 



FIG. 1 



FIG. 2 



FIG. 3 



transmitted undiminished to all parts of the enclosing vessel, and 

 must be added to the pressure due to the weight of the liquid. 



The atmospheric pressure is the external pressure due to the 

 weight of the air, and may be taken to have an average value 

 of 14.7 Ib. per sq. in. 



Buoyant Effort. When a solid body is immersed in a liquid, 

 a buoyant effort equal to the weight of the liquid displaced acts 

 upwards and opposes the action of gravity. The weight of a 

 body, as shown by a scale, is decreased by an amount equal to 

 the buoyant effort, that is, by an amount equal to the weight 

 of liquid displaced. This principle is called the principle of 

 Archimedes, from the name of its discoverer. 



SPECIFIC GRAVITY 



The specific gravity of a body is the ratio between its weight 

 and the weight of a like volume of distilled water at a tempera- 

 ture of 39.2 F. The weight of 1 cu. ft. of water at 39.2 F., 

 which is the temperature of its maximum density, is 62.425 Ib. 

 For nearly all engineering purposes 62.5 Ib. is used as an approx- 

 imate value. 



Since a column of water 1 sq. in. in cross-section and 1 ft. 

 high is T*T cu. ft., its weight is 62.5-=- 144 = .434 Ib. 



The accompanying table gives the specific gravities and 

 weights per cubic inch of a great variety of substances. 



