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 Ibs. 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 w r orked 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 Ibs. ; 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 compact, 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 1 6 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 



