132 THEORY OF CONSTRUCTION AND SCIENTIFIC DESCRIPTION 



known; yet it is the first of the class only, or that marked (106), 

 which becomes available in practice, and for this reason, that the 

 power of the press, or the aggregate pressure which it is capable of 

 exciting, is known d priori, or immediately assignable from the con- 

 ditions of construction, while the load upon the safety valve, and the 

 force with which the plunger descends, have each to be determined 

 by calculations founded on circumstances connected with the aggre- 

 gate or ultimate pressure. 



140. Referring to equation (105), which has been purposely inves- 

 tigated, for expressing the intensity of pressure on a square inch of 

 surface, and multiplying both sides by r -\- t the denominator of the 

 fraction, we shall have 



nr -f- nt~ct, 



from which, by transposing and collecting the terms, we get 



(c n) t n r ; 



then by division, the value of #, or the thickness of metal in the 

 cylinder to withstand the pressure, becomes 



.,nr 

 -^T* (109). 



From which it appears, that if a constant value adapted to practical 

 purposes, can be assigned to n, the rule for" calculating the thickness 

 of metal in the cylinder will become exceedingly simple. 



Now, it has been remarked by several eminent practical engineers, 

 as well as by the most approved and intelligent manufacturers, that 

 the extreme pressure on a square inch of the piston,* should never 

 exceed half the cohesive power of the material ; but according to Dr. 

 Robison, the cohesive power of cast iron of a medium quality is equal 

 to 16648 Ibs. ; hence we have 



therefore, if 8324 Ibs. be adopted as the limit of pressure upon a 

 square inch of surface, the foregoing value of t becomes 

 - 8324 r 

 "16648 8324" 



consequently, in order that the strain produced by the pressure may 

 not exceed the elastic power of the material ; 



* There is no occasion to limit the pressure to the piston only, since every square 

 inch of surface in contact with the fluid sustains the same pressure. This limitation 

 has frequently caused a misapprehension respecting the mode of ascertaining the 

 pressure on an inch of surface. 



