lower disc must be made smaller than the upper one, and therefore the pressure on the upper one, being in proportion to its size, has a constant tendency to close the valve. As it is of the greatest importance that a throttle-valve should remain closed after steam is shut off, and never be opened at any time accidentally, the arrangement described accomplishes just what is needed--that is, makes the valve work comparatively easily, and at the same time keeps it closed after the steam has been shut off. [Illustration: Fig. 85. Scale ³⁄₄ in. = 1 foot.] [Illustration: Fig. 86. Scale ³⁄₄ in. = 1 foot.] QUESTION 156. _How is the valve opened and closed?_ _Answer._ By a lever, _O′ O_, called a _throttle-lever_, figs. 85, 86[41] and 71. This lever is connected by a rod, _d_, called the _throttle-stem_, with a bell crank, _i_, the other arm of which works the rod _c_, to which the throttle-valve is attached. The rod _d_ works through a steam-tight stuffing-box, _f_, in the back end of the boiler. The end of the throttle-lever is attached to two links, _g_, fig. 86, which are fastened by a pin to the stud _h_. These links have a slight vibratory motion, which enables the pin _k_, by which the lever _O′ O_, is fastened to the rod _d_, to move in a straight line, which is necessary in order that the rod _d_ may work steam-tight in the stuffing-box, _f_. The throttle-lever has a latch, _l_, which gears into a curved rack, _n_, so as to hold the lever and valve in any required position. This latch is operated by a trigger, _m_. Various other devices are used to fasten the throttle-lever and thus hold it in any position required. [41] Fig. 86 is a plan, showing the _top_ of the valve and lever. QUESTION 157. _How are the steam pipes constructed?_ _Answer._ The steam, after it is admitted by the throttle-valve, as was explained in answer to Question 155, passes into the throttle-pipe _I_ and the dry-pipe _m m_, fig. 41. At the front end of the dry-pipe a pipe, _n_, figs. 40 and 41, which divides into two branches like the top of the letter ~T~ and is therefore called a T-pipe, is attached. The steam-pipes _o_, _o_, fig. 40, are connected to each of the two branches of the T-pipe at one end and to the cylinders at the other.[42] [42] In fig. 40 the right-hand side represents a section through the steam-pipe _o o_, and the left a section through the exhaust pipe _e e_. [Illustration: Fig. 87. Scale 1¹⁄₂ in. = 1 foot.] [Illustration: Fig. 88. Scale 1¹⁄₂ in. = 1 foot.]