576

MODERN STEAM PRACT1CE.

is less or greater. For it is a law of nature, that bent tubes underinternal pressure have always a tendency to become straight. Thetube has an oval section, is fixed at one end, and in communi-cation with the steam in the boiler; the other end is filled up, andhas a connecting rod attached to a lever, with toothed quadrantand pinion, the pinion spindle carrying the index handle, whichrevolves in the centre of a disc plate, marked round the circumfer-ence to indicate the pressure in pounds per square inch, the wholebeing inclosed in a suitable box. It is fitted to the cab, or weatherprotecting plate, having a u-shaped wrought-iron pipe connection.Water lodging in the bend, or oil poured in, prevents such delicateinstruments from becoming overheated with the steam so necessaryfor their proper action.

The steam whistle is placed at the back of the outside fire box,or on the manhole door on the top of the box. It consists of aplug tap screwed into the door, which is in connection with anannular chamber, the steam escaping through a narrow annularaperture, blowing against the edge of the bell, which is generallyfrom 3 to inches in diameter and 2% inches in depth. It isusually cast in hard brass, this size producing the musical noteD natural, which is sounded with about 8o lbs.of steam pressure, higher pressures giving amuch shriller sound; and as the fulness oftone is affected by the pressure, the drivermust open the aperture cautiously, or wire-draw the steam through the tap to give thefull note when the pressure exceeds 8o lbs.When the handle is not within reach of thedriver, a lever is substituted, with a con-necting rod and handle passing through theweather plate, fitted to the back of the firebox.

The guard’s whistle is of the same con-struction, but the bell is made larger, about3^ to 4 inches in diameter and about 5 inchesin depth. This gives a much lower note,A,V ““ Bei“ which is easily distinguished from the ordin-ary whistle. Instead of the plug tap, a valvehaving a screwed spindle is sometimes used, which is worked witha suitable handle, or lever and connecting rod. This plan has the

Fig. 430.—Guard’s Steam Whistle.