434 THE STEAM-ENGINE. 



to the water in the cold cistern, and have therefore a tendency to raise the 

 temperature of that water. To prevent this, a pump called the cold pump, 

 represented at L in fig. 8, is provided. By this pump water is raised from any 

 convenient reservoir, and driven through proper tubes into the cold cistern. 

 This cold pump is wrought by the engine, the rod being attached to the beam. 

 Water being, bulk for bulk, heavier the lower its temperature, it follows that 

 the water supplied by the cold pump to the cistern will have a tendency to 

 sink to the bottom, pressing upward the warmer water contained in it. A 

 waste-pipe is provided, by which this water is drained off, and the cistern 

 therefore maintained at the necessary temperature. 



From what has been stated, it is also evident that the hot well B, into which 

 the warm water is thrown by the air-pump, will receive considerably more 

 water than is necessary to feed the boiler. A waste-pipe, to carry off this, is 

 also provided ; and the quantity necessary to feed the boiler is pumped up by 

 a small pump, O, the rod of which is attached to the beam, as represented in 

 fig. 8, and which is worked by the engine. The water raised by this pump is 

 conducted to a reservoir from which the boiler is fed, by means which will be 

 hereafter explained. 



We shall now explain the manner in which the machine is made to open 

 and close the valves at the proper times. By referring to the explanation 

 already given, it will be perceived that at the moment the piston reaches the 

 top of the cylinder, the upper steam- valve G must be open, to admit the steam 

 to press it down ; while the exhausting valve I must be opened, to allow the 

 steam to pass to the condenser ; and the condensing valve E must be opened, 

 to let in the water necessary for the condensation of the steam ; and at the 

 same time the lower steam-valve H must be closed, to prevent the passage of 

 the steam which has been admitted through G. The valves G, I, and E,must 

 be kept open, and the valve H kept closed, until the piston arrives at the bottom 

 of the cylinder, when it will be necessary to close all the three valves, G, I, 

 and E, and to open the valve H, and the same effects must be produced each 

 time the piston arrives at the top and bottom of the cylinder. All this is ac- 

 complished by a system of levers, which are exhibited in fig. 8. The pivots 

 on which these levers play are represented on the framing of the engine, and 

 the arms of the levers G 7 , H 7 , and I 7 , communicating with the corresponding 

 valves G, H, and I, are represented opposite a bar attached to the rod of the 

 air-pump, called the plug-frame. This bar carries certain pegs and detents, 

 which act upon the arms of the several levers in such a manner that, on the 

 [ arrival of the beam at the extremities of its play upward and downward, the 

 levers are so struck that the valves are opened and closed at the proper times. 

 It is needless to explain all the details of this arrangement. Let it be sufficient, 

 as an example of all, to explain the method of working the upper steam-valve 

 G. When the piston reaches the top of the cylinder, a pin strikes the arm of 

 the lever G 7 , and throws it upward : this, by means of the system of levers, 

 pulls the arm of the valve G downward, by which the upper steam-valve is 

 raised out of its seat, and a passage is opened from the steam-pipe to the cyl- 

 inder. The valve is maintained in this state until the piston reaches the bottom 

 of the cylinder, when the arm G 7 is pressed downward, by which the arm G 

 is pressed upward, and the valve restored to its seat. By similar methods the 

 levers governing the other three valves, H, I, and E, are worked. 



The valves used in these engines were of the kind called spindle-valves. 

 They consisted of a flat circular plate of bell metal, A D, fig. 10, with a round 

 spindle passing perpendicularly through its centre, and projecting above and 

 below it. This valve, having a conical form, was fitted very exactly, by 

 grinding into a corresponding circular conical seat, A B C D, fig. 11, which 



