These important physical circumstances are now only indicated in a general way. As we proceed with our account of the invention and improvement of the steam engine, they will be developed more fully and accurately.
(17.)
In the case of the apparatus of De Caus ([5].), the heat of the fire acting on the vessel D C ([fig. 2.]) will raise the temperature of the water contained in it, and also of the air confined within it above the surface of that water. This air, as it is increased in temperature, will also increase in elasticity; it will therefore press on the surface of the water with increased force, and will gradually force the water upwards in the tube; and this effect would continue until all the water in the vessel would be forced up the tube.
But at the same time that the heat acting on the vessel increases the temperature of the air above the water, it also produces a partial evaporation of the water, so that more or less steam is mixed with the air in the vessel above the surface [Pg031] of the water; and this steam possessing elasticity, unites with the air in pressing on the surface of the water, and in raising it in the tube.
Figs. 4, 5, and 6.
Let us now revert to the brief account of the engine of the Marquis of Worcester, described in "The Century of Inventions." We collect from that description that the vessel in which the water was evaporated was separate from those which contained the water to be elevated; also that there were two vessels of the like description, the contents of which were alternately elevated by the pressure of the "water rarefied by the fire;" in other words by steam; and that the water was raised in an uninterrupted stream, by the management of two cocks communicating with these vessels and with the boiler. The following is such an apparatus as would answer this description. Let E ([fig. 4.]) be the vessel containing the water to be evaporated, placed over a proper furnace A; let S be a pipe to allow the steam produced from the boiling water in E to pass into the vessels where its mechanical action is required. Let R represent a cock or regulator, having in it a curved passage, leading from S to the tube T, when the lever or handle L is in the position represented by the cut; but leading to the tube T′, when the lever L is turned one quarter of a revolution to the right, as represented in [fig. 5.] By the shifting of this lever, therefore, the steam pipe S may be made to communicate alternately with the tubes T and T′. The tubes T and T′ are carried respectively to two vessels V and V′, which are filled with the water required to be raised. In these [Pg032] vessels tubes enter at C and C′, descending nearly to the bottom: these tubes have valves at B and B′, opening upwards, by which water will be allowed to pass into the vertical tube F, but which will not allow it to return downwards, the valves B and B′ being then closed by the weight of the water above them.
Let G G′ be a pipe entering the sides of the vessels V and V′, for the purpose of filling them with the water to be raised: let K be a cock having a curved passage similar to the cock R, and leading to a tube by which water is supplied from the reservoir or other source from which the water to be raised is drawn. When the cock K is placed as represented in [fig. 4.], the water from the reservoir will flow through the curved passage in the cock K into the tube G′, and thence into the vessel V′; but when this cock is turned one quarter round, by shifting the lever to the left, it will take the position represented in [fig. 6.], and the water will flow through the curved passage into the tube G, and thence into the vessel V. Let us now suppose the vessel V already filled with water to be elevated, and the vessel V′ to have discharged its contents. The cock R is turned, so as to allow the steam generated in the boiler E to pass into the tube T, and thence into the upper part of the vessel V, while the cock K is turned so as to allow the water from the reservoir to pass into the tube G′, and thence into the vessel V′. The steam collecting in the upper part of the vessel V′ presses with its elastic force on the surface of the water therein, and forces the water upwards in the tube C; it passes through the valve B, which it opens by the upward pressure received from the action of the steam, and thence into the tube F, its descent into the tube C′ being prevented by the valve V′, which can only be opened upwards. As the steam is gradually supplied from the boiler E, the water in the vessel V is forced up the tube C, through the valve B, and into the tube F, until all the contents of the vessel V above the lower end of the tube C have been raised. In the meanwhile, the vessel V′ has been filled with water, through the cock K: when this has been accomplished, the man who attends the machine shifts the cocks R and K, so as to give them the position represented in [fig. 5.] and [fig. 6.] [Pg033] In this position, the steam from the boiler, being excluded from the tube T, will be conducted to the tube T′, and thence to the vessel V′, while the water from the reservoir will be excluded from the tube G′, and conducted through the tube G to the vessel V. The vessel V will thus be replenished and, by a process similar to that already described, the contents of the vessel V′ will be forced up the tube C′, through the valve B′, and into the tube F; its descent into the tube C being prevented by the valve B, which will then be closed. After the contents of the vessel V′ have thus been raised, and the vessel V replenished, the two cocks R and K are once more shifted, and the contents of V raised while V′ is replenished, and so on.