250 
OF THE PARTS OF 
[sect. VII. 
ance, and were it always provided, there would be little chance of accident if the 
valves be properly constructed and attended to. 
It becomes insufficient in high pressure boilers, because a common low pressure 
boiler contains about ten times the volume of steam required for one stroke of the 
engine, consequently the time of twenty strokes must elapse before the density of 
the steam could accumulate to three times its working density, supposing the 
engine to be stopped, and the valve out of order; but if the boiler contains only as 
much steam as is required for one stroke, the force will be increased to three times 
in the time the engine would have made two strokes. This rapidity of the increase 
of force does not leave the necessary time to examine, nor even to open the valves 
in this extreme case, and the hazard must be greater in consequence. In all cases 
the time of accumulating power should not be shorter than it is in the common 
boiler. Besides, in working an engine where the excess of force increases so fast, 
the loss of steam would be considerable from any variation of the heat of the fire, 
even were the valve to act properly, and therefore there is a temptation to load the 
valve beyond its regular weight. To render the security on the stoppage of the 
engine equal in all cases, the excess of strength should be inversely as the space 
allowed for steam. 
It is still more important to consider the subject, in relation to the danger arising 
from unequal action of the fire; and for this the excess of strength should be 
inversely as the contents of the boiler expressed in units of the power. 
Thus, taking the horse power as the measure, if one boiler contains twenty cubic 
feet for each horse power, and another only ten, the boiler with only ten feet of 
space should be of twice the strength ; for equal powers require equal fires, and the 
effect of excess of fire in raising the temperature and force of the steam is inversely 
as the quantity of matter acted upon ; hence the risk of the dangerous increase of 
strength is inversely as the quantity of water and steam the boiler contains. 
523. The proportion for excess of strength I shall therefore consider to be two 
times that which is proper for the working pressure when the boiler contains twenty 
cubic feet for each horse power; and containing any other quantity as n cubic feet 
per horse power, it will be 
40 
n : 20 : : 2 : - . 
n 
The effect of unequal expansion, of improper form and flexure, and of wear, 
must be included in the calculation of the strength ; for these are not allowances 
for risk, but actually necessary for security. 
Boilers may fail from strains produced by other causes besides the force of the 
steam, and these may be noticed to guard against the circumstance which produces 
them.