BOILERS FOR STATIONARY ENGINES.

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strain. Professor W. R. Johnson, of the Franklin Institute ofAmerica, whose inquiries into the strength of cylindrical boilersare of great value, may be quoted as an authority:—

“ ist. To know the force which tends to burst a cylindrical boilerin the longitudinal direction, or, in other words, to separate the headfrom the curved sides, we have only to consider the actual area ofthe head, and to multiply the units of surface by the number ofunits of force, applied to each superficial unit, this will give the totaldivellent. To counteract this, we have, or may be conceived to have,the tenacity of as many longitudinal bars as there are units in thecircumference of the cylinder. The united strength of these barsconstitutes the total retaining or quiescent force, and at the momentwhen rupture is about to take place the divellent and quiescentforces must obviously be equal.

“2d. To ascertain the amount of force which tends to rupture thecylinder along the curved side, or rather along the opposite sides, wemay consider the pressure as applied through the whole breadth of thecylinder upon each lineal unit of diameter. Hence the total amountof force which would tend to divide the cylinder in halves, by sepa-rating it along two lines of opposite sides, would be represented bymultiplying the diameter by the force exerted on each unit of surface,and this product by the length of the cylinder. But even withoutregarding the length, we may consider the force requisite to rupturea single band in the direction now supposed, and of one lineal footin breadth, since it obviously makes no difference whether the cylinderbe long or short, in respect to the ease or difficulty of separating thesides. When the diameter of a. boiler is increased, it must be bornein mind that the area of the ends is also increased, not in the ratioof the diameter, but in the ratio of the square of the diameter; andit will be seen, that instead of the force being doubled, as in the caseof the direction of the diameter and circumference, it is quadrupledupon the ends, or, what is the same thing, a cylinder double thediameter of another cylinder, has four times the pressure in thelongitudinal direction. The retaining force, or the thickness of metalof a cylindrical boiler, does not, however, increase in the same ratioas the area of the circle, but simply in the ratio of the diameter,consequently the thickness of the metal will require to be increased,in the same ratio as the diameter is increased. From this it appearsthat the tendency to rupture, by blowing out the ends of a cylindri-cal boiler, will not be greater in this direction than it is in any other