LOCOMOTIVE ENGINE. 
421 
compresses the spring in the balance more, and increases the pressure upon the 
valve, so that the free egress of the surplus steam is checked, and the pressure of 
the steam is allowed to become greater than that indicated by the balance when the 
valve is shut; the longer the lever is, the greater is this difference of pressure, and 
it is sometimes as much as 10 lbs. per square inch. 
In observing the pressure on the safety valve, allowance must also be made for 
the effect of the mitre, or conical part of the valve; for when it is raised, the steam 
acts on the conical part as well as on the bottom of the valve, and has therefore a 
greater power in lifting the valve ; and this diminution of the pressure on the valve 
varies with the extent of the mitre and with the degree that the valve is opened. 
These different circumstances render the safety valve but an imperfect means of 
ascertaining the pressure of the steam in the boiler. In stationary engines, which are 
generally worked at a much lower pressure, a mercurial gauge is often used to 
indicate the pressure of the steam; but this instrument cannot be used in a loco 
motive, as a tube of great size, and not less than twelve feet high would be re 
quired ; it has, however, been used as a means of testing the accuracy of the in 
dications of the safety valve by a temporary connection with the engine. 
0, (Plates LXXXIX. and XC.,) is the lock-up safety valve, enclosed in a case, to 
prevent access to it so as to increase the pressure to a dangerous degree. The 
valve is exactly similar in construction to the other safety valve, but instead of 
being held down by a lever and spring balance, several small elliptical springs, x x, 
(Plate XC.,) about six inches long, are placed one above another and over the valve, 
and pressed down by a screw at the top in the frame y y, fixed into the valve seat. 
By turning this screw, the pressure on the valve can be adjusted to any required 
degree; and when the case is fixed on, the valve is effectually protected from having 
the pressure altered. The lock-up valve is loaded rather more heavily than the 
ordinary working pressure, 50 lbs.; so that it does not blow except when the pres 
sure has exceeded that limit, as in performing work requiring more power than 
usual. 
A large spiral spring is used in some engines to press upon the valve, being fixed 
in a similar manner to the elliptical springs; it is rather more contact, but is not quite 
so free in its action, as the pressure increases more rapidly on the using of the valve. 
Man Hole.—P, (Plates LXXXIX. and XC.) is a circular opening into the boiler, 
called the man hole; it is 16 inches in diameter, and surrounded by a ring, z, (Plate 
XC.,) bolted on to the boiler, having a flanch at the top for fixing on the cover. 
This opening is large enough for a man to enter, and affords access to the interior of 
the boiler for making repairs in it, or for cleaning it out. 
Mud Holes.—These are two small openings, Q Q, in both sides of the fire-box at 
the bottom, closed by plates bolted upon the outside, and are for the purpose of