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 be remembered that in the single-acting engine the process of condensation was suspended while the piston ascended in the cylinder, and therefore the play of the jet of cold water in the condenser was stopped during this interval. In the double-acting engine, however, the flow of steam from the cylinder to the condenser is continued, whether the piston ascends or descends, and therefore a constant condensation of steam must be produced. The condensing jet, therefore, does not in this case, as in the former, play with intervals of intermission. A constant jet of cold water must be maintained in the condenser. 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. [Illustration: 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. [22] Farey, Treatise on the Steam Engine, p. 122. [23] Farey on the Steam Engine, p. 297. [Pg193] [Illustration: DOUBLE-ACTING ENGINE.—CITY SAW-MILLS.] CHAP. VIII.