Boiler Explosions. 159 



testing with cold water has not rendered the weakest point of the 

 vessel much stronger.. 



As soon as a boiler is in use, the agents of destruction incident 

 thereto begin their work. Probably chief among these, is the 

 steam itself. The unit of elasticity, by which the expansive force 

 of elastic fluids is measured, is for popular use, one pound on one 

 square inch of surface. We glance at a steam guage and the little 

 hand may indicate fifty. Let us ascertain what that means. If 

 a boiler is twelve feet long and three feet in diameter (very com- 

 mon dimensions) and contains thirty-four three inch lubes, the 

 tvv"0 heads with tube surface deducted have remaining 1,864 

 square inches. The cylinder of the boiler contains 16,280, equal- 

 ing in all 18,150 square inches which, multiplied by fifty pounds 

 pressure, give a total of nearly one million pounds, or a fraction 

 over 450 tons, continually tending lo rend the cylinder. Boilers 

 are made round or approximately so, for two reasons. It is the 

 cheaper form and one naturally self-supporting. I say approxi- 

 mately round, for they are not a true circle and cannot be made 

 so owing to the lap of the longitudinal seams. Now this enorm- 

 ous pressure, tends to force the shell of the boiler to a true circle. 

 The pressure is never constant. Great and unequal strains are pro- 

 duced along the under edge of the lap, which vary from time to 

 time according to the different degree of pressure. In effect it is 

 similar to bending a piece of iron back and forth in the hands, 

 only on a more minute scale. In time the same result will be 

 effected, destruction of the fibre of the iron. 



Many purchasers of these steam generators commit the serious 

 mistake of selecting boilers of insufficient capacity, simply because 

 one or two hundred dollars cheaper. In so doing, the door is 

 opened through which many dollars will pass in the way of fuel 

 without an adequate return. But when a boiler has just the ca- 

 pacity to supply the demand by forcing the fires, a nearly full 

 opening of all passages to the engine will result. The steam flows 

 rapidly through them, twice at every revolution of the engine, 

 this flow is suddenly and positively checked. While so checked, 

 there is a rapid accumulation of steam from the forced fires. The 

 boiler expands to the greatest limit in retaining the increasing 



