134 THEORY OF CONSTRUCTION AND SCIENTIFIC DESCRIPTION 



EXAMPLE 15. What is the diameter, and what the thickness of 

 metal, in a press of 300 tons power ? 



By the second rule above, we have 

 D 2 = 300 ~ 2.9186 102.81 nearly ; 

 therefore, by extracting the square root, we obtain 



D= V 102.81 = 10.13 inches; 



consequently, according to the remark under the equation (109), the 

 thickness of metal is 



t = 10.13 4- 2 = 5.065 inches. 



143. The rules by which the preceding examples have been resolved, 

 are very nearly, but not precisely the same as those employed by 

 Messrs. Bramah in the construction of their excellent presses ; the only 

 difference, however, consists in their assuming a higher number as 

 the limit of pressure, the standard which they employ being 8556 Ibs. 

 upon a square inch of the piston, thereby indicating, that they reckon 

 on a higher cohesive power in the material, than that which we have 

 adopted as the basis of our theory. 



Now, 8556 Ibs. on a square inch, is equivalent to 6619.8824 Ibs. 

 upon a circular inch ; whereas the constant which we have chosen is 

 only 6537.6696 Ibs., being a difference of 82.2128 Ibs. upon the 

 circular inch, a difference that need not be regarded in practice, as 

 the error will always fall on the side of safety, giving a smaller power 

 to the press than what it really possesses. 



144. It sometimes, indeed it very frequently happens, that presses 

 are constructed, without any attention being paid, to the relation which 

 subsists between the strength of the parts, and the strain which they 

 have to resist ; in all such cases, therefore, it may be interesting to 

 possess a rule, by which the merits or demerits of a press so con- 

 structed can be ascertained, for in this way a failure in the instrument 

 may be prevented, and a remedy applied to any defect that may 

 exist. 



Now, according to the first equation of class (104), the pressure 

 upon a square inch is 



P 



-.~7854D~" 



and according to equation (105), it is. 

 ct 



