_Answer._ By having two eccentrics, _J_, _J_, Plate III (also shown in Plate II,) for each cylinder. One of these is fixed or _set_ on the shaft in such a position as to move the valve so that the engine will run in one direction; the other eccentric is set so that the engine will run the reverse way. The ends of the two eccentric rods are attached to what is called a _link_, _L_, (Plates II and III,) the object of which is to furnish the means of quickly engaging and disengaging either eccentric rod to or from the rocker, _K_. The link is operated by a system of levers, consisting of the lifting shaft, _M_, and arms, _N_, _N_, and the reverse lever, _O_, _O_, (Plate II). The principles and working of these will be more fully explained hereafter. QUESTION 77. _What are the principal parts or “organs” of a locomotive boiler?_ _Answer._ 1. A fire-place, or, as it is called a _fire-box_, _G_, (Plate II,) which is surrounded with water. 2. A cylindrical part, _P P_, (Plates I and II,) attached to the fire-box at one end and to a chamber, _Q_, called the _smoke-box_, at the other. 3. The tubes or flues _a a′_, (Plate II and fig. 40,) which connect the fire-box with the smoke-box and pass through the cylindrical part of the boiler and are surrounded with water. 4. The smoke-stack or chimney _R R_. QUESTION 78. _What is each of these parts or organs for and of what do they consist?_ _Answer._ The fire-box _G_ furnishes the room for burning the fuel, and consists of an inner and outer shell made of boiler plate, with the space between the two filled with water; a grate, _b b_, (Plate II,) formed of cast-iron bars, with spaces between them for admitting air for the combustion of the fuel, which is placed on the top of them; a door, _C_, called the _furnace-door_, for supplying the grate with fuel; a receptacle, _d d_, below the grate, to collect ashes, and therefore called the _ash-pan_, which is supplied with suitable dampers, _n′_, _n′_, for admitting or excluding the air from the fire. The cylindrical part _P P_, or _waist_ of the boiler as it is sometimes called, contains the greater part of the water to be heated. The flues or tubes, as they are generally called, of which a locomotive has from one to two hundred, are usually two inches in diameter, and about eleven feet long. They conduct the smoke and products of combustion from the fire-box to the smoke-box. These tubes are made of small diameter so as to sub-divide the smoke into many small streams and thus expose it to a large radiating surface through which the heat is conducted to the water. The smoke-stack serves partly for removing into the open air the smoke which passes through the flues, and partly for producing a strong draft of air, which is indispensably necessary for the rapid combustion of the fuel, and also for collecting and extinguishing the sparks from the fire. QUESTION 79. _How is the draft produced in locomotive boilers?_