SECT. VII.] 
STEAM ENGINES. 
249 
on a circular inch ; hence, 
In square or circular plates it becomes 
3*81 p r 
For wrought iron, 0’006 r = t when low pressure steam is to be confined, and 
both r and t are in inches; when the length is great compared with the breadth, 
or the bounding edges are not properly confined in one direction, double the result. 
Of the Excess of Strength to render Boilers safe. 
522. The pressure tending to separate a boiler is about proportional to the 
load on the safety valve; that to crush it together is equal to the pressure of the 
atmosphere. In the latter case it cannot exceed that pressure; in the former a 
considerable excess may take place if any derangement happens to the valves ; and 
it is to provide against accident, in the event of the valves being out of order, that 
an excess of strength in the boiler is necessary. 
It is clearly a matter of opinion, founded on the experience of past accidents, as 
to the degree of excess required, and it has been almost universally allowed, that 
three times the pressure on the valve in the working state should be borne by the 
boiler without injury. 
This degree of excess of power seems to be fully sufficient for the ordinary low 
pressure steam boilers ; indeed, I should think twice the pressure a proper allow- 
in this manner, but that the pressure is limited. 
1 f e 
For wrought iron, / = 17800, and e = ; hence = 3-33 ; and therefore we have, 
for the greatest stress in lbs. on a circular inch that a plate will bear. 
When the plate is either square or circular, it becomes 6-85 lbs. per circular inch, = 8-5 lbs. per 
square inch. When of other proportions, as in the equation, and when the length is very consi 
derable, it becomes simply 3-33 lbs. per circular inch, or 4-25 lbs. per square inch. 
Copper bears about the same strain.