OF THE HYDROSTATIC PRESS. 131 



Therefore, if for c in each of the preceding expressions, we substitute 

 its value as determined by experiment, and which for cast iron, accord- 

 ing to Dr. Robison, is 16648 pounds avoirdupois upon a square inch ; 

 then we shall have 



,_ Pr * 



- 13076 D a P' (106). 



A 107 )- 



-130763 s w (108). 



The following example will illustrate the use of these equations, the 

 value of t the thickness of the metal coming out the same by each. 



138. EXAMPLE 13. What must be the thickness of metal in the 

 cylinder of a Hydrostatic Press, to resist a pressure of 30000 Ibs. ; 

 the diameter of the cylinder being 5 inches, that of the forcing pump 

 one inch, and of the safety valve one fourth of an inch ; being the 

 same dimensions which we have employed in the preceding examples? 



Here we have given P = 30000 Ibs. ; D = 5 inches ; and conse- 

 quently, r 2J inches; therefore, by substitution, equation (106) 

 gives 



30000 



n . , 



'= 13076 X 5*- 30000 = '^ "^ bem * 

 thing more than one fourth of an inch. 



In order that the entire pressure on the piston of the cylinder, may 

 be equal to 30000 Ibs. according to the conditions of the question ; 

 the force with which the plunger of the forcing pump descends, must 

 be equal to 1200 Ibs. ; therefore, by equation (107), we have 



before. 



Again, in order that the entire pressure may be equal to 30000 Ibs. 

 the weight upon the safety valve must be 75 Ibs. ; hence, from equa- 

 tion (108), we obtain 



the two cases foregoing. 



139. It may not be improper here to remark, that although the 



requisite thickness of metal is alike assignable from either of the 



above equations, when the respective pressure and diameters are 



* Where the constant number 13076 = 16648 X .7854. 

 K2 



