250 OF THE PARTS OF [SECT. VH. 



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 



. : 20 :: 2 : - . 



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. 



