SECT. VII.] 
STEAM ENGINES. 
241 
For head links it becomes, 
gj v p = b t = the breadth multiplied by the thickness in inches. 
504. Rule. Multiply the diameter of the steam piston in inches, by the 
square root of twice the elastic force of the steam in the boiler, in lbs. per circular 
inch, and the product divided by 84 is the diameter of the rod in inches. 
Example. If the force of the steam be 16 lbs. per circular inch, and the diameter 
of the cylinder 54 inches, then the square root of 32 is 5*657, and 
54-X_5 657 = 3*6 inches, the diameter required. 
For the atmospheric pressure it is one-sixteenth of the diameter. 
505. Of the strength of rods alternately extended and compressed. In 
the compression of rods the force increases with the flexure; but if the length never 
exceed about thirty-six times the diameter, its error will be very small to assume that 
degree of flexure ; and by taking in addition the greatest possible deviation, from 
misfitting, which is half the diameter of the rod, 1 with this simplification we have 
_ a 2 / __ _ /8-75 P 
D 2 P = 
8-75 
i /8*75 P \ 1. 
nearly, or a = D ^—/ / ' 
D 
For cast iron /= 15300, and a = P. 
For malleable iron f— 17800, and a = 
^VP. 
45 
D 
For tempered steel / = 45000, and a = ^ VP. 
This rule applies to piston rods of double engines, parallel motion rods, air pump 
and force pump rods, and the like ; and if P be increased in the ratio of the radius 
to the sine of the greatest angle a connecting rod makes with the direction, it 
applies to connecting rods. 
506. Rule. Multiply the diameter of the piston in inches by the square root 
of twice the pressure of the steam in lbs. on a circular inch, and divide the 
product by, 
42 for cast iron, 
45 ,, wrought iron, 
72 „ tempered steel; 
the quotient will be the diameter in inches. 
Example 1. The force of the steam being 16 lbs. per circular inch, and the 
diameter of the piston 80 inches, that of the piston rod should be, for wrought iron, 
80 x V32 
45 
10*06 inches. 
Example 2. The force of the steam being 4 atmospheres = 46 lbs. per circular 
1 Practical Essay on Cast Iron, art. 246. 
2 h