BOILERS FOR STATIONARY ENGINES.

2 9

resistance, as that when one part is at the point of rupture, everyother part is on the point of yielding to the same uniform force.This appears to be an important consideration in mechanical con-structions of every kind, as any material applied for the security ofone part of a vessel subject to uniform pressure, whilst another partis left weak, is so much material thrown away; and in stationaryboilers, or in moving bodies such as locomotive engines and steamvessels, they are absolutely injurious, at least so far as the parts aredisproportionate to each other, because when maintained in motionthey become an expensive and unwieldy encumbrance. The greaterportion of the fire-boxes in locomotive boilers have the rectangularform, and in order to economize heat, and give space for the furnace,it becomes necessary to have an exterior and interior shell. Thatwhich contains the furnace is generally made of copper, firmly unitedby rivets, and the exterior shell, which covers the fire-box, is madeof iron, and united by rivets in the same way as the copper fire-box.Now these plates would of themselves, unless supported by rivettedstays, be totally inadequate to sustain the pressure. In fact, with one-tenth of the pressure, the copper fire-box would be forced inwardsupon the furnace, and the external shell bulged outwards, and withevery change of force these two flat surfaces would move backwardsand forwards, like the sides of an inflated bladder, at the point ofrupture. To prevent this, and give the large flat surfaces anapproximate degree of strength with the other parts of the boiler,wrought-iron or copper stays, 1 inch in diameter, are introduced.They are first screwed into the iron and copper on both sides toprevent leakage, and then firmly rivetted to the exterior and interiorplates. These stays are from 6 inches to 4^ inches asunder, form-ing a series of squares, and each of these will resist a strain of about15 tons before it breaks. Let us suppose the greatest pressure con-tained in the boiler to be 200 lbs. on the square inch, and we have6x6x200=7200 lbs., or 3tons, the force applied to a squarecontaining 36 square inches. Now as these squares are supportedby four stays, each capable of sustaining 15 tons, we have 4x15 =60 tons as the resisting powers of the stays; but the pressure is notdivided amongst all the four, but each stay has to sustain that pres-sure, consequently the ratio of strength to the pressure will be 4^to 1 nearly, which is a very fair proportion for the resisting powerof that part.

We have treated of the sides, but the top of the fire-box and