560 



THE STEAM-ENGINE. 



of brass, and cast with a flange at the bottom to attach it to the boiler. The 

 mitred surface of the valve is ground into the valve-seat, so as to rest in steam- 

 tight contact with it. Across the valve-seat, which is two and a half inches 

 in diameter, is cast a thin piece D, seen in plan in fig. 83, and in section in 

 fig. 82, which extends from the top to the bottom, and has a longitudinal hole 

 through it, in which the spindle B of the valve works : by this hole it is guided 

 when it rises from its seat. A projection E is cast upon the seat of the valve, 

 in which a standard F is inserted. This standard is forked at the top, and re- 

 ceives the end of a lever G, which turns in it upon a centre. A rod H is 

 jointed to this lever by another pin at three inches from the former, and the 

 lower end of this rod, ground to a point, presses upon the centre of the valve 

 A. At the other end of the lever, which is broken off* in fig. 82, at a distance 

 of three feet from the centre pin, inserted in the fork of the pillar F, the rod of 

 a common spring-balance w, fig. 71, is attached by a finger-nut n. The bottom 

 of this spring-balance is secured on to the fire-box. This balance is screwed 

 up by the finger-nut on the valve-lever until the required pressure on the lever 

 is produced through the medium of the rod H, this pressure being generally 

 fifty pounds per square inch above the atmosphere. When the pressure of the 

 steam in the boiler exceeds this, the valve A is raised from its seat, and the 

 steam escapes. 



It is evident that the sliding-weight by which the pressure of the safety 

 valve is sometimes regulated in stationary engines would not be admissible in 

 a locomotive-engine, since the motion of the engine would constantly jolt it up 

 and down, and cause the steam to escape. One of the disadvantages attending 

 the use of the spring-valve is, that it can not be opened to let the steam escape 

 without increasing its force, so that the steam, when escaping, must really 

 have a greater pressure than that to which the valve has been previously ad- 

 justed. The longer the lever is, the greater will be this difference of pres 

 sure, inasmuch as a given elevation of the pin governing the rod H would 

 cause a proportionally greater motion in that end of the lever attached to the 

 spring. 



The second safety-valve O is enclosed in a case, so that it is inaccessible, 

 and its purpose is to limit the power of the engineer to increase the pressure 

 of steam in the boiler. This valve is similar in construction to the former, but 

 instead of being held down by a lever, is pressed upon by several small ellip 

 tical springs placed one above another over the valve, and held down by a 

 screw which turns in a frame Y, fixed into the valve-seat. By this screw the 

 pressure on the valve can be adjusted to any required degree ; and if the open 

 safety-valve be screwed down to a greater pressure, the steam will begin to 

 escape from this second valve. 



Also in the case where the boiler produces surplus steam faster than its es 

 cape can be effected at the valve N, the pressure will sometimes be increased 

 until the valve O is opened, and its escape will take place from both valves. 



The whole weight of the engine bears upon those parts of the six axles K/, 

 fig. 69, which project beyond the wheels. Boxes are formed in which these 

 parts of the axles turn, and through the medium of which the weight of the engine 

 rests upon them. Over these boxes are constructed oil or grease cups, by means 

 of which the axles are constantly lubricated. It is usual to lubricate the axles of 

 the engine itself with oil : the axles of the tender, and other coaches and wag- 

 ons, are lubricated with a mixture of oil and tallow. In the middle of the box 

 in which the axle turns, and between the two oil-cups, is cast a socket, in 

 which the end of the spindle on which the spring presses rests. The springs 

 are composed of a number of steel plates, laid, in the usual manner, one above 

 the other, increasing in length upward. In the engine here described, the 



