was below the piston, passing up the passage a, escapes through the tube P to 

 the condenser. When the piston has descended, the rod E moves the slide 

 downward, so as to give it the position represented in fig. 31. The steam in 

 T H has now access to the bottom of the cylinder, while the steam above the 

 piston passing through P escapes to the condenser. In this way the operation 

 of the piston is continued and the steam consumed at each stroke only exceeds 

 the capacity of the cylinder by what is necessary to fill the passages between 

 the slide and the cylinder. 



In a slide constructed in this manner, the steam filling the space T H has a 

 tendency to press the slide back, so as to break the contact of the rubbing 

 surfaces, and thereby to cause the steam to leak from the space T H to the 

 back of the slide. This is counteracted by the packing x, at the back of 

 the slide. 



In engines of very long stroke, the extent of the rubbing surfaces of slides 

 of this kind renders it difficult to keep them in steam-tight contact and to in- 

 sure their uniform wear. In such cases, therefore, separate slides, upon the 

 same principle, are provided at the top and bottom of the cylinder, moved, 

 however, by a single rod of communication. 



In slides, as we have here described them, the same motion which admits 

 steam to either end of the cylinder, withdraws it from the other end. Such 

 an arrangement is only compatible with the operation of a cylinder which 

 works without expansion ; for in such a cylinder the full flow of steam to the 

 piston is only interrupted for a moment during the change of position of the 

 slide. But if the steam act expansively, it would be necessary to move the 

 slide, so as to stop its flow to one end of the cylinder, without at the same time 

 obstructing the escape of steam from the other end to the condenser. It would 

 therefore be necessary that the slide should close the passage leading to the 

 cylinder at one end, without at the same time obstructing the communication 

 between the passage from the cylinder to the condenser at the other end. On 

 the arrival of the piston, however, at the bottom of the cylinder, it would be 

 necessary immediately to put the lower passage to the cylinder in communica- 

 tion with the steam-pipe, and the upper passage in communication with the 

 condenser. This would necessarily suppose two motions of the slide as well 

 as some modifications in its length. Let the length of the slide be such that 

 when the passage to the top of the cylinder is stopped, the lower part of the 

 slide shall not reach the passage to the lower part of the cylinder ; and let 

 such a provision be made in the mechanism by which the rod E governing 

 the slide is driven that it shall receive two motions during the descent of the 

 piston, the first to be imparted to it at the moment the steam is to be cut off, 



