548 THE STEAM-ENGINE. 



with the passages m and n, fig. 69, leading to the top and bottom of the cylin- 

 der: these are called the steam-ports. They also communicate with a passage 

 o leading to the mouth of a curved horizontal pipe p' connecting the front ends 

 of the two cylinders, as seen in figs. 69, 74. These curved pipes unite in a 

 single vertical pipe p, called the blast-pipe, seen in figs. 67, 74, this vertical 

 pipe becomes gradually small toward the top, and terminates a little above the 

 base of the funnel or chimney G. In the valve-box is placed the slide-valve 

 v to which is attached the spindle /'. This spindle moves through a stuffing- 

 box k f , and is worked by gearing, which will be described hereafter. Accord- 

 ing to the position given to the slide, a communication may be opened between 

 the steam-chest, or the waste-port, and either end of the cylinders. Thus 

 when the slide is in the position represented in fig. 67, the steam-chest com- 

 municates with the front end of the cylinder, while the waste-port communi- 

 cates with the hinder end. If, on the other hand, the spindle V being pressed 

 forward, move the slide to its extreme opposite position, the steam-port n would 

 communicate with the waste-port 0, while the steam-chest would communicate 

 with the steam-port m, steam would, therefore, be admitted to the hinder end 

 of the cylinder, while the foremost end would communicate with the waste- 

 port. It will be perceived that this arrangement is precisely similar to that 

 of the slide-valves already described. The slide-valve is represented on a 

 larger scale in fig. 76, where A is the hinder steam-port, B the foremost steam- 

 Fig. 76. 



port, and C the waste-port. The surfaces D, separating the steam-ports from 

 the waste-ports, are called the bars : they are planed perfectly smooth, so that 

 the surfaces F and G of the slide-valve, also planed perfectly smooth, may 

 move in steam-tight contact with them. These surfaces are kept in contact 

 by the pressure of the steam in the steam-chest, by which the slide-valve is 

 always pressed down. In its middle position, as represented by the dotted 

 lines in the figure, both the steam-ports are stopped by the slide-valve, so that 

 at that moment no steam is admitted to either end of the cylinder. On either 

 side of this intermediate position the slide has an inch and a half play, which 

 is sufficient to open successively the two steam-ports. 



The cylinders are inserted at one end in the plate of the smoke-box, and at 

 the other in the tube-plate of the boiler. They are closed at either end by 

 cast-iron covers, nearly an inch thick, flanged on by bolts and screws. In the 

 cover of the cylinder attached to the tube-plate is a stuffing-box, in which the 

 piston-rod plays. The metallic pistons used in locomotive engines do not dif- 

 fer materially from those already described, and therefore need not be here 

 particularly noticed. From their horizontal position they have a tendency to 

 wear unequally in the cylinders, their weight pressing them on one side only ; 

 but from their small magnitude this effect is found to be imperceptible in prac- 

 tice. In the engine here described the stroke of the piston is eighteen inches, 

 and this is the most usual length of stroke in locomotive engines. The piston, 

 in its play, comes at either end within about half an inch of the inner surface 

 of the covers of the cylinders, this space being allowed to prevent collision. ] 

 In the foremost cover of the cylinder is inserted a cock q*, figs. 67, 69, by 



