Chap. XXT. STEAM CONSIDERED AS A MOVING POWER.

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that when the engine is at rest, it is as represented. The cocks being all open andthe steam issuing copiously from the boiler, and no condensation going on at L, the steammust drive out all the air, and at lastfollow it through the valve Q*: onshutting the valves b and d , and open-ing the valve S of the condenser,condensation will immediately takeplace : at this time there is no pres-sure on the under side of the pistonat A, and it immediately descends.

The communication between the lowerpart of 11 and the upper part of Abeing open, the steam will go fromB into the space left by the pis-ton of A : it must therefore expand,its elasticity diminishing, and willno longer balance the pressure ofthe steam above the piston of B.

This piston therefore, if not withheldby the beam, will descend until it is inequilibrio, having steam of equal den-sity above and below it; but it cannotdescend so far, for the cylinder A iswider than B : and the arm of the beamat which thepistonhangs is longer thanthe arm which supports the piston ofB ; therefore when the piston of B hasdescended as far as the beam willpermit it, the steam between thepistons occupies a larger space than itdid when both pistons were at the topsof their cylinders; and its densityas well as its elasticity diminishingas its bulk increases, it is no longera balance for the steam on the upperside of B, which piston pulls at the beam with all the difference of their pressures:as the pistons descend, the steam that is between them grows continually rarer and lesselastic, and both pistons pull the beam downwards.

When each has reached the bottom of the cylinder, if the cock a and the eduction cock atthe bottom of A be shut, and the cocks b and d be opened, the pistons will arrive at the top: thecylinder B is now filled with steam of the ordinary density, and the cylinder A with an equalabsolute quantity of steam, but expanded into a larger space : on shutting the cocks b and d,and opening the cock a, and the eduction cock at the bottom of A, condensation will again beproduced, and the pistons descend: thus the operation is continued. When botl pistons are atthe top of their cylinders the active pressure on the piston at B is nothing, while that on thepiston of A is equal to the full pressure of the atmosphere on its area : this multiplied bythe length of the arm by which it is supported gives its mechanical energy :as the pistons descend, the pressure on the piston of B increases, whilethat on the piston A diminishes; when both are at the bottom, the pres-sure on the piston of B is at its maximum.

Boilers -Much yet appears to be wanting in practice to enable us to

construct a perfect boiler, and science has not succeeded in determining thebest form it should have : early in their application to the steam-engine, theywere of a spherical shape, in consequence of that figure having the largestcapacity with the least material; it was also fancied to be the strongest: aboiler of this form, placed upon an open fire, required a large quantity of fuelto raise a small quantity of steam, the heat being carried away by the sur-rounding air, and one of the first improvements was setting it in brickwork,or some non-conducting material that would bear the effect of the fire. Theflues were so constructed that the flame, after passing below the sphere,wound round the sides in a spiral direction, and then passed off to thechimney, a damper being introduced to regulate the draught: this gaveway to the cylindrical boiler, which was in a short time improved bymaking the top hemispherical, and arching the bottom upwards in orderthat a larger surface might be offered to the effects of the fire ; the sideswere inclined in such a way that the flame acted in a slight degree uponthem.

Fig. 1904. hornbloweh’s dot’ble cylinder.

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Fig.19tf.