The piston will, therefore, be pressed upward by the action of the steam below 

 it, against the vacuum above it, and will ascend with the same force as that 

 with which it had descended. 



This alternate action of the piston upward and downward 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. 14, 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 when the piston has arrived at the bottom 

 of the cylinder, as represented in fig. 15, 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 cylin- 

 der, 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 upward or downward, 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 cyl- 

 inder and its escape to the condenser. 



It will be remembered that in the single-acting engine the process of con- 

 densation was suspended while the piston ascended in the cylinder, and there- 

 fore 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 ascend or descend, 

 and therefore a constant condensation of steam must be produced. The con- 

 densing jet, therefore, does not in this case, as in the former, play with inter- 

 vals 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 inanu- 



VOL. II.— 29 



