This alternate action of the piston upwards and downwards may evidently be continued by opening and closing the valves alternately in pairs. Whenever the piston is at the top of the cylinder, as represented in [fig. 33.], the valves S and C′, that is, the upper steam-valve and the lower exhausting-valve, are opened, and the valves C and S′, that is, the upper exhausting-valve and the lower steam-valve, are closed; and [Pg191] when the piston has arrived at the bottom of the cylinder, as represented in [fig. 34.], the valves C and S′, that is, the upper exhausting-valve and the lower steam-valve, are opened, and the valves S and C′, that is, the upper steam-valve and the lower exhausting-valve, are closed.
If these valves, as has been here supposed, be opened and closed at the moments at which the piston reaches the top and bottom of the cylinder, it is evident that they may be all worked by a single lever connected with them by proper mechanism. When the piston arrives at the top of the cylinder, this lever would be made to open the valves S and C′, and at the same time to close the valves S′ and C; and when it arrives at the bottom of the cylinder, it would be made to close the valves S and C′, and to open the valves S′ and C.
If, however, it be desired to cut off the steam before the arrival of the piston at the termination of its stroke, whether upwards or downwards, then the steam-valves must be closed before the arrival of the piston at the end of its stroke; and as the exhausting-valve ought to be left open until the stroke is completed, these valves ought to be moved at different times. In that case separate levers should be provided for the different valves. We shall, however, return again to the subject of the valves which regulate the admission of steam to the cylinder and its escape to the condenser.
(117.)
It will presently appear that in the double-acting engine applied to manufactures, the motion of the piston was subject to more or less variation of speed, and the quantity of steam [Pg192] admitted to the cylinder was subject to a corresponding change. The quantity of steam, therefore, drawn into the condenser was subject to variation, and required a considerable change in the quantity of cold water admitted through the jet to condense it. To regulate this, the valve or cock by which the water was admitted into the condenser was worked in the double-acting engine by a lever furnished with an index, by which the quantity of condensing water admitted into the condenser could be regulated. This index played upon a graduated arch, by which the engine-man was enabled to regulate the supply.
HEATHFIELD HOUSE, NEAR BIRMINGHAM, THE RESIDENCE OF WATT.
FOOTNOTES:
[20] These effects are explained in my Treatise on Heat; and they have lately been verified by experiments made with locomotive engines by M. de Pambour, who found that the steam raised from the boiler of a locomotive engine, under a pressure of above 50 lbs. per square inch, was in the state of common steam as it issued from the chimney at a very diminished pressure and at a lower pressure.
[21] It is strange that this absurdity has been repeatedly given as unquestionable fact in various encyclopædias, as well as in by far the greater number of treatises expressly on the subject.