_Answer._ Constant use is better than occasional use. Having two injectors on the same engine, one on each side, the one on the runner’s side will be used while running. The one on the other side should be used when standing. All pipe connections must be tight, so as to prevent the leaking of air. The pipe carrying steam to the instrument should be from such part of the boiler as will insure the use of dry steam, and the waste pipe must not be contracted. The instrument represented in the engraving is the injector of 1876, manufactured by Messrs. William Sellers & Co. of Philadelphia. Beside this, Mack’s and several other kinds of injectors are now used. [Illustration: Fig. 71. Scale, ³⁄₈ in. = 1 foot.] LIST OF PARTS DESIGNATED BY LETTERS OF REFERENCE IN FIG. 71. _A_, Furnace Door. _B_, _B_, Driving Wheels. _C_, Driving Axle. _D_, _D_, Suction Pipes. _E_, Ash Pan Damper. _F_, _F_, Foot Steps for getting on and off the Locomotive. _G_, _G_, Hand Holds for getting on and off the Locomotive. _H_, _I_, Cab. _J_, _J_, Doors in front of Cab. _K_, _K_, Windows in front of Cab. _L_, Steam Gauge. _M_, Spring Balance. _N_, Steam Gauge Lever. _O′ O_, Throttle Lever. _P_, Water Gauge. _Q_, Stand for Tallow Can. _R_, Drip Pipe for Gauge Cock. _T′ T_, Rod for operating Feed Cock. _T′_, Regulator for Feed Cock. _U V_, Reverse Lever. _W_, Whistle. _X_, Blow-Off Cock. _Z_, _Z_, Frames. _a_, _a_, Heater Cocks. _a a′_, Heater Pipe. _b_, Blower Cock. _c_, _c_, Oil Cups for oiling Main Valves. _d_, Handle for opening Valves in Sand Box. _e_, _e_, Handles for opening Pet Cocks. _f_, Handle for opening Cylinder Cocks. _g_, Whistle Lever. _h′_, Whistle Handle. _h_, Rod connecting Whistle Handle to Whistle Lever. _j_, Handle for left hand Feed Cock. _m m_, Lever for shaking Grate Bars. _n_, Bell Crank for opening front Ash Pan Damper. _o_, _o_, Check Chains. _p_, Pipe for carrying off water from Gauge Cocks. _s_, _s_, _s_, _s_, Gauge Cocks. _w_, Handle for opening Blow-Off Cock. QUESTION 143. _How can the height of the water in the boiler be known?_ _Answer._ Two appliances are used by which the height of the water in the boiler can be observed. These are: 1. _Gauge_ or _try cocks_. 2. The _glass water gauge_. [Illustration: Fig. 72. Scale ¹⁄₄.] [Illustration: Fig. 73. Scale, 3 in. = 1 foot.]