When the arms are in the position represented in fig. 83, the drop-door is held up in the place in which it is shown; but when they are turned as in fig. 84, the door falls down so that the burning coal can be taken out of the opening at _G_, and, by raising up the ash-pan damper, _H_, fig. 84, can be raked out on the track or into suitable pits usually provided for this purpose. The drop-doors are sometimes perforated so as to admit air to the fuel on top of them. The grates for burning anthracite coal usually have about four solid wrought-iron bars between that number of tubes. These bars can be withdrawn, and the fire then falls into the ash-pan through the opening left by the withdrawal of the tubes. QUESTION 154. _How are the dampers of the ash-pan operated?_ _Answer._ They are connected by suitable rods and levers with two handles, _l_, _l_, fig. 71, which are raised or lowered, thus opening or closing the dampers. PART IX. THE THROTTLE-VALVE AND STEAM PIPES. QUESTION 155. _How is the steam admitted to and the supply regulated or shut off from the cylinders?_ _Answer._ By a valve, _h_, fig. 41, called a _throttle-valve_, which is usually placed at the end of the pipe _I_, near the top of the dome. Throttle-valves are sometimes placed in the smoke-box at the front end, _n_, fig. 41, of the dry-pipe. Until within a few years they consisted of plain slide-valves which covered openings similar in form to the steam-ports, but smaller in size. The pressure on such valves is of course greatest when there is no steam underneath, which is the case when the valves are closed. It is then very difficult to open them, and as it is important that the supply of steam admitted to the cylinders when the locomotive is started should be easily regulated, such valves are objectionable, and therefore the form has been introduced which is illustrated in fig. 41, and also on a larger scale in fig. 85, which represents a longitudinal section of the throttle-pipe and valve. This is what is called a _double-poppet valve_, and consists of two circular discs, _a_ and _b_, which cover two corresponding openings in the end of the pipe _I_. When these discs are raised up, as shown in fig. 85, steam flows in around their edges, as represented by the darts. It will be observed that the steam pressure in the boiler comes on top of the disc _a_ and against the under side of _b_. The pressure on the one thus neutralizes or balances that on the other. If the two discs were of the same size, the pressure of the one would be exactly the same as on the other; but as they are joined together and are made to fit steam-tight on their seats by what are called _countersunk joints_, their diameters must be somewhat larger than the openings they cover. The only practicable way, therefore, by which the lower disc _b_ can be introduced into the end _h_ of the pipe _I_ so as to cover the lower opening is through the upper opening _a_. For this reason the