_Answer._ In this country it is usually placed over the fire-box, but in Europe it is placed further forward, either about the centre of the boiler or near the front end of the tubes. Sometimes two domes are used on engines, in this country, one over the fire-box and another near the front end. QUESTION 117. _How is the steam conducted from the dome to the cylinders?_ _Answer._ By a pipe _I m m_, fig. 41, called the _dry-pipe_, which extends from the top of the dome to the front tube-plate. On the front side of the tube-plate and inside the smoke-box two curved pipes, _O_, fig. 41, (shown also in fig. 40,) called _steam-pipes_, are attached to the dry-pipe at one end, and to the cylinders at the other. The vertical portion of the dry-pipe in the dome, sometimes called the _throttle-pipe_, is usually made of cast iron, the horizontal part of wrought iron, and the steam pipes of cast iron. QUESTION 118. _How is the loss of heat from locomotive boilers by radiation and convection prevented?_ _Answer._ By covering the boiler and dome with wood, called _lagging_, about ⁷⁄₈ inch thick, which is a poor conductor of heat, and then covering the outside of the wood with Russia iron, the smooth, polished surface of which is a poor radiator of heat. QUESTION 119. _What is the smoke-box for?_ _Answer._ The smoke-box _Q_ is simply a convenient receptacle for the smoke before it escapes into the chimney or smoke-stack, which is attached to the top of the smoke-box. It also affords a convenient place for the steam and exhaust pipes, where they are surrounded with hot air and smoke, and not exposed to loss of heat by radiation. The front end of the smoke-box is usually made of cast iron, with a large door in the centre which affords access to the inside. QUESTION 120. _How are the chimneys or smoke-stacks of locomotives constructed?_ _Answer._ The forms of smoke-stacks which have been used are almost numberless. For burning bituminous coal and wood they are generally made with a central pipe, _R_, fig. 41, and a conical-shaped cast-iron plate, _S_, called the _cone_ or _spark deflector_, which, as the latter name implies, is intended to deflect the motion of the sparks and cinders which are carried up with the ascending current of smoke and air in the pipe _R_, so as to prevent them from escaping into the open air while they are incandescent, or “alive.” A wire netting, _t t_, is also provided, which is intended as a sort of sieve to enclose the sparks and cinders, and at the same time allow the smoke to escape. The receptacle _h h_ is intended as a chamber in which the burning cinders will be extinguished before they escape. For burning anthracite coal, a simple straight pipe, without a deflector or wire netting, is ordinarily used. QUESTION 121. _What are the proportions and materials usually employed in the construction of smoke-stacks?_