This arrangement, however, was soon laid aside for one much more simple and obvious. This consisted in the production of exactly the same effect by a single cylinder in which steam was introduced alternately above and below the piston, being at the same time withdrawn by the condenser at the opposite side. Thus the piston being at the top of the cylinder, steam is introduced from the boiler above it, while the steam in the cylinder below it is drawn off by the condenser. The piston, therefore, is pressed from above into the vacuum below, and descends to the bottom of the cylinder. Having arrived there, the top of the cylinder is cut off from all communication with the boiler; and, on the other hand, a communication is opened between it and the condenser. The steam which has pressed the piston down is therefore drawn off by the condenser, while a communication is opened between the boiler and the bottom of the cylinder, so that steam is admitted below the piston: the piston, thus pressed from below into the vacuum above, ascends, and in the same way the alternate motion is continued. Such is the principle of what is called the _Double-acting Steam Engine_, in contradistinction to that described in the last chapter, in which the steam acts only above the piston while a vacuum is produced below it. It is evident that, in the arrangement now described, the condenser must be in constant action: while the piston is descending the condenser must draw off the steam below it, and while it is ascending, it must draw off the steam above it. As steam, therefore, must be constantly drawn into the condenser, the jet of cold water which condenses the steam must be kept constantly playing. This jet, therefore, will not be worked by the valve alternately opening and closing, as in the single engine, but will be worked by a cock, the opening of which will be adjusted according to the quantity of cold water necessary to condense the steam. When the steam is used at a low pressure, and, therefore, in a less compressed state, less condensing water would be necessary than when it is used at a higher pressure and in a more compressed state. In the one case, therefore, the condensing cock would be less open than in the other. Again, the quantity of condensing water must vary with the speed of the engine, because the greater the speed of the engine, the more rapidly will the steam flow from the cylinder into the condenser; and, as the same quantity of steam requires the same quantity of condensing water, the supply of the condensing water must be proportional to the speed of the engine. In the double-acting engine, then, the jet cock is regulated by a lever or index which moves upon a graduated arch, and which is regulated by the engineer according to the manner in which the engine works.