_Answer._ The throw of the valve. This will be clear if we refer to fig. 146, which represents a valve with a travel of 5¹⁄₂ in. It will be seen that when it is in the extreme position in which it is shown the width _A_ of the opening of the exhaust-port is very small. If this opening is contracted too much it will of course interfere with the free escape of the exhaust steam. It is therefore best to make the exhaust-port so wide that with the greatest travel of the valve the width of its opening will be either quite or very nearly equal to the width of the steam-port. [Illustration: _Fig. 146._ Scale ³⁄₁₆ in. = 1 foot.] [Illustration: Fig. 148. Fig. 147. Scale ³⁄₄ in. = 1 foot.] QUESTION 240. _Where is the reverse lever located and how is it constructed?_ _Answer._ It is located on the _foot-board_[65] _K′ K′_, as shown in plate II. It consists of a lever _O_, _O_, with the fulcrum at the lower end. The _reverse-rod e k_, which connects the lever with the vertical arm _k_ of the lifting-shaft, is attached above the fulcrum of the reverse lever. Figs. 147 and 148 represent side and end views of the lever on an enlarged scale and with some of the details attached which are omitted on plate II. _C_, _C_ are two curved bars, which in this country are usually called _quadrants_, but in England are called (and more properly) _sectors_. These are placed on each side of the reverse-lever and are fastened to some portion of the engine. They have notches, _n_, _n_, _n_, cut in them to receive the _latch L_, which slides in a clamp, _H_, and holds the reverse-lever in the notches in which it is placed. This latch is operated by a _trigger_, _D_, which is grasped by the locomotive runner when he takes hold of the handle _A_ of the reverse-lever. The trigger works on a pin, _E_, as a fulcrum and is attached to the latch by a rod, _r r_. When the trigger is pressed up against the handle, the latch is raised out of the notches by the rod _r r_, and is pressed into them again by the spring _s_ when the trigger is released. _F_ is a set-screw which presses against a gib, _G_, and is intended to keep the latch tight and prevent the reverse-lever from shaking. [65] The _foot-board_ _K′ K′_, plates 2 and 3, is a platform for the locomotive runner and fireman to stand on and is located at the back end of the engine. QUESTION 241. _How are the notches in the sector arranged?_