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STRENGTH OF BOILER.

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the rectangular chamber are driven together, distorting the cylinder into an ellipse and rupturing the side stays, 1'. In practice, the case in the locomotive is not as bad as is here shown, because the roof stays are not all vertical, and those towards the sides being inclined reduce the amount

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of strain which would otherwise come upon the horizontal screw stays.

I am of opinion, that both the girder tays and the suspending stays are open to objection. To get rid of these objections the Belgians and, I believe, the French have introduced a class of fire-box (see fig. 13) wherein the outer shell is made rectangular, and wherein the horizontal staying of that shell therefore must be con

tinued up to the very top, and then the top of the internal fire-box may be stayed, by pendent roof stays from the outer box, without any disturbance of the balance of pressures.

Very frequently locomotive boilers are furnished with steam chests, K (fig. 14). If a hole be made in the barrel the full size of the steam chest, obviously there is a large reduction in the strength of the barrel at that part. If (as is sometimes done) only a small hole be made, there is still a very considerable reduction, because, as will at once be seen, the pressures above and below the piece of plate at the bottom of the steam chest being equal, that plate can only operate to tie the barrel of the boiler together by the strength which it possesses as a stay-bar, for which purpose it is very unfitted, as it is of a curved form. The proper way to guard against loss of strength from the use of steam chests is to place strong straight bars, L, across the cavity, and to put a strengthening ring, M, for attachment of the bar ends (see fig. 14). This application of a strengthening ring surrounding a hole should be borne in mind wherever man-holes or other holes of considerable size are made.

The next element in the security of the boiler, and it is an important element, is the safety valve.

Looking at the high pressure employed and at the jolting of the engine as it runs along, a weighted valve would obviously be inadmissible, and, therefore, from the very early days, locomotive valves have been held down by springs. The common arrangement is to have a mere lever with a Salter's balance spring at the end, generally so graduated that the figures on the balance, after allowing for the leverage, the area, and the weight of the valve and lever, represent pounds on the square inch. Objections are urged against this valve (and other spring

TYPES OF SAFETY VALVES.

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valves) on several grounds. One is, that it can be interfered with by the driver, who may screw it down more and more; this, however, can always be guarded against by proper stops. Mr. Ramsbottom's simple arrangement, shown in diagram 15, overcomes this objection, and gets rid of any question of variation of pressure

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produced by the act of the engine driver. Here it will be seen there is only a single central spring N, which holds down a pair of valves v v by means of a cross head o, one end of which is prolonged and formed into the handle P, through which either valve may be made the 'point d'appui' for opening the other valve by pressure, either upwards or downwards, as the case may be, upon the handle.

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TYPES OF SAFETY VALVES.

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Salter balance, and that is, the difficulty of obtaining an aperture sufficient to deliver a large quantity of steam, without at the same time materially increasing the pressure, because, obviously, as the valve rises the spring is distended or compressed,

and thereby the load is increased, while there is good reason to believe that at the same time the effective pressure to act upon the underside of the valve is reduced. There are valves in use (fig. 16) which get rid of this head of objection, either by applying the pressure of the spring N through. a lever Q, the angle of which varies as the valve rises, so as to give the spring, although further distended, no more effective pressure upon the valve v, or by so constructing the valve and seating (fig. 17) that, when the valve v lifts, a margin of the valve exterior to the steam-tight seating, shall come into play in aid of the lifting surface. Very satisfactory results have been obtained from both constructions. It must be borne in mind, however, that in the locomotive, which has its fire urged by the waste steam blast, there is the special advantage of an automatic cessation of the strong production of steam at

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FIG. 17. Adams' safety valve.

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