420 
STEPHENSON’S PATENT 
prevent the roof of the fire-box being uncovered and getting burnt. The glass tube 
is, however, the more certain guide, being less affected by the oscillations of the 
water than the gauge cocks. 
The Lead Plug, m, (Plates XC. and XCII.,) described before, is an additional 
security against any accident arising from the water being suffered to get too low 
in the boiler. 
Safety Valves.—The pressure of steam in the boiler is regulated by the safety 
valve N, (Plates LXXXIX., XC., and XCII.,) the construction of which is shewn in 
fig. 9 and 10, drawn to a scale of inches to a foot, or three times the size of the 
engraving. Fig. 9, is a longitudinal section through the valve, and fig. 10, a plan of 
the valve-seat with the valve removed. The valve A is made of brass ; it is 
conical round the edge, or mitred at an angle of 45°, and has 
a spindle or stalk, B, cast on it in the middle. The seat 
C of the valve is also of brass, and is cast with a flange 
at the bottom, to bolt it on to the boiler; and the valve 
is ground into the upper part, so as to fit it steam-tight. 
The opening in the valve seat, C, is 2-^ inches diameter, 
and across it is cast a thin piece D, extending to the 
bottom, and having a longitudinal hole through it, in 
which the spindle B of the valve works; this is to hold 
the valve steady when it is raised, and to guide it into its 
seat again. A projecting lug, E, is cast on the valve 
seat, in which is fixed the standard, F; this is forked at 
the top, and receives the end of the lever G, which turns in it upon a centre pin; a 
rod, H, is jointed to the lever by another pin, at 3 inches from the former one, and 
bears directly upon the valve. 
At the other end of the lever, and at a length of 3 feet from the fulcrum, is 
attached by a finger nut, the rod of a common spring balance, w, (Plates LXXXIX. 
and XCII.,) the bottom of which is fixed on to the fire-box ; this spring balance is 
screwed up by the finger nut on the valve lever, until the required pressure on the 
valve is produced, which is generally 50 lbs. on the square inch above the at 
mosphere; and the valve, on rising to let out the surplus steam, has to raise the 
spring balance, which acts upon it with twelve times the leverage. 
In stationary engines, the safety valve is kept down by a weight hanging on the 
lever, and shifted to different positions to alter the pressure on the valve; but in a 
locomotive engine a weight could not be used, because the motion of the engine 
would cause it to jolt up and down, and the valve would be continually letting off 
steam. There is one disadvantage attending the use of the spring balance that the 
other plan is free from; namely, that any opening of the valve by raising the lever,