BOILERS FOR STATIONARY ENGINES.

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the cylindrical plates of the large boiler. Now this is not the casewith the other parts of the boiler, as the circumference of a cylinderincreases only in the ratio of the diameter, consequently the pressureinstead of being increased in the ratio of the squares of the diameter,as shown in the ends, is only doubled, the circumference of the6 feet boiler being twice that of the 3 feet boiler.

Let us, for the sake of illustration, suppose the two cylindricalboilers such as we have described to be divided into a series of hoopsof 1 inch width, and taking one of these hoops in the 3 feet boiler, weshall find it exposed at a pressure of 40 lbs. on the square inch to aforce of 1440 acting on each side of a line drawn through the axis ofa cylinder 36 inches diameter and 1 inch in depth, and which line formsthe diameter of the circle. Now this force causes a strain tending toburst the hoops in the 3 feet circle of 720 lbs., and assuming the pres-sure to be increased until the force becomes equal to the tenacity orretaining power of the material, it is evident, in this state of the equi-librium of the two forces, that the preponderance on the side of theinternal pressure would insure fracture; and supposing we take theplates of which the boiler is composed, of one quarter of an inch thick,and the ultimate strength at 34,000 lbs. on the square inch, we shall

have = 472 lbs. per square inch, as the bursting pressure of theboiler. Again, as the forces in this direction are not as the squares,but simply as the diameters, it is clear that at 40 lbs. on the squareinch we have in a hoop an inch in depth, or that portion of a cylinderwhose diameter is 6 feet, exactly double the force applied to rendthe iron asunder, as in the 3 feet boiler. Now, assuming the platesto be quarter of an inch thick, as in the 3 feet boiler, it follows, ifthe forces at the same pressure be doubled in the large cylinder, thatthe thickness of the plates must also be doubled, in order to sustainthe same pressure with equal security; or, what is the same thing,the 6 feet boiler must be worked at half the pressure, in order tosecure the same degree of safety as attained in the 3 feet boiler at thegiven pressure. From these facts it may be useful to know thatboilers having increased dimensions, should also have increasedstrength in the ratio of their diameters; or, in other words, the platesof a 6 feet boiler should be double the thickness of the plates of a3 feet boiler, and so on as the diameter increases.

The relative powers of force applied to cylinders of different dia-meters become more strikingly apparent when we reduce them totheir equivalents of strain per square inch, as applied to the ends