408 



THE STEAM-ENGINE. 



the interior of the Vessel a vacuum. The valve B will be kept closed by the 

 atmospheric pressure. But the elastic force of the air between the valve A 

 and the surface of the water in the well, or reservoir, will open A, so that a 

 part of this air will rush in and occupy the vessel V. The air in the suction- 

 pipe S, being thus allowed an increased space, will be proportionably dimin- 

 ished in its elastic force, and its pressure will no longer balance that of the 

 atmosphere acting on the external surface of the water in the reservoir. This 

 pressure will therefore force water up in the tube S, until its weight, together 

 with the elastic force of the air above it, balances the atmospheric pressure. 

 When this has taken place, the water will cease to ascend. 



Let us now suppose, that, by shifting the regulator, the communication is 

 opened between T and the boiler, so that steam flows again into V. The 

 condensing-cock G being removed, the vessel will be again heated as before, 

 the air expelled, and its place filled by the steam. The condensing-pipe be- 

 ing again allowed to play upon the vessel V, and the further supply of steam 

 being stopped, a vacuum will be produced in V, and the atmospheric pressure 

 will force the water through the valve A into the vessel V, which it will 

 nearly fill, a small quantity of air, however, remaining above it. 



Thus far the mechanical agency employed in elevating the water is the 

 atmospheric pressure, and the power of steam is no further employed than 

 in the production of a vacuum. But, in order to continue the elevation of the 

 water through the force-pipe F, above the level of the steam-vessel, it will be 

 necessary to use the elastic pressure of the steam. The vessel V is now 

 nearly filled by the water which has been forced into it by the atmosphere. 

 Let us suppose, that, the regulator being shifted again, the communication be- 

 tween the tube T and the boiler is opened, the condensing-cock removed, and 

 that steam flows into V. At first, coming in contact with the cold surface of 

 the water and that of the vessel, it is condensed ; but the vessel is soon heated, 

 and the water formed by the condensed steam collects in a sheet or film upon 

 the surface of the water in V, so as to form a surface as hot as boiling water.* 

 The steam then being no longer condensed, presses on the surface of the 

 water with its elastic force ; and when that pressure becomes greater than the 

 atmospheric pressure, the valve B is forced open, and the water issuing through 

 it, passes through E into the force-pipe F ; and this is continued until the 

 steam has forced all the water from V and occupies its place. 



The further admission of steam through T is once more stopped by moving 

 the regulator, and the condensing-pipe being again allowed to play on V, so as 

 to condense the steam which fills it, produces a vacuum. Into this vacuum, 

 as before, the atmospheric pressure will force the water and fill the vessel V. 

 The condensing-pipe being then closed, and steam admitted through T, the 

 water in V will be forced by its pressure through the valve B and tube E into 

 F, and so the process is continued. 



We have not yet noticed the other steam vessel V', which, as far as we 

 have described, would have remained filled with common atmospheric air, the 

 pressure of which on the value A 7 would have prevented the water raised in 

 the suction-pipe S from passing through it. However, this is not the case ; 

 for, during the entire process which has been described in V, similar effects 

 have been produced in V', which we have only omitted to notice to avoid the 

 confusion which the two processes might produce. It will be remembered, 

 that after the steam, in the first instance, having flowed from the boiler through 

 T, has blown the air out of V through B, the communication between T and 

 the boiler is closed. Now, the same motion of the regulator which closes 

 this, opens the communication between T' and the boiler ; for the sliding- 

 * Hot water, being lighter than cold, floats on the surface. 



