BOILERS FOR STATIONARY ENGINES.

2 7

To pursue the subject a little further, let us suppose the pressure tobe 450 Ibs. on the square inch, which a well-constructed boiler of thisdescription will bear before it bursts, and we have the enormous forceof 29,754 tons, or nearly 30,000 tons, compressed within a cylinder30 feet long and 6 feet diameter. This is, however, inconsiderablewhen compared with the locomotive and some marine boilers, which,from the number of tubes they contain, present a much larger surfaceto pressure. Locomotive boiler engines are usually worked at120 lbs. on the square inch; and taking one of the usual constructionwe shall find that it rushes forward on the rail with a pent-upforce within its interior of nearly 60,000 tons, which is ratherincreased than diminished at an accelerated speed. In a station-ary boiler, charged with steam at a given pressure, it is evidentthat the forces are in equilibrium, and the strain being the samein all directions, there will be no tendency to motion. Supposing,however, this equilibrium to be destroyed, by accumulative pressure,tili rupture ensues, it follows that the forces in one directionhaving ceased, the others in an opposite direction, being active,would project the boiler from its seat with a force equal to thatwhich is discharged through the orifice of rupture. The directionof motion would depend upon the position of the ruptured part: ifin the line of the centre of gravity, motion would ensue in that direc-tion ; if out of that line, an oblique or rotatory motion round thecentre of gravity would be the result. (An explosion of a plainvertical boiler may be taken as an example: it gave way at thebottom of the fire-box or bottom of the boiler, and by the reactive forceof the steam it was lifted about 100 feet in the air like a sky-rocket,and when the force was spent, and the water and the steam expelled,it descended, landing on the identical spot where it had rested pre-vious to the explosion.) The momentum or quantity of motion pro-duced in one direction would be equal to the intensity or quantitylost; and the velocity with which the body would move would bein the ratio of the impulsive force, or the quantity lost. Therefore,the quantity of motion gained by an exploded boiler in one directionwill be as the weight and quantity lost in that direction. Thesedefinitions, however, belong more to the province of the mathemati-cian, and may be easily computed from well-known formuke on thelaws of motion.

The following table shows the bursting pressure of boilers, aslikewise the safe working pressure, as deduced from experiment,