2122. The tubes used for conveying steam may be formed of the same metals as the boilers; but cast iron is now generally used. Earthen or stoneware tubes have been tried; but it is extremely difficult to prevent the steam from escaping at their junctions. The tubes are laid along or around the house or chamber to be heated, much in the same manner as flues, only less importance is attached to having the first course from the boiler towards the coldest parts of the house, because the steam-tube is equally heated throughout all its length. As steam circulates with greater rapidity, and conveys more heat in proportion to its bulk, than smoke or heated air, steam-pipes are consequently of much less capacity than smoke-flues, and generally from 3 to 6 inches in diameter inside measure. Where extensive ranges are to be heated by steam, the pipes consist of two sorts, mains or leaders for supply, and common tubes for consumption or condensation. Contrary to what holds in circulating water or air, the mains may be of much less diameter than the consumption pipes, for the motion of the steam is as the pressure; and, as, the greater the motion, the less the condensation, a pipe of one inch bore makes a better main than one of any larger dimensions. This is an important point in regard to appearance as well as economy. In order to procure a large mass of heated matter M'Phail and others have proposed to place the pipes in flues, where such exist. They might also be laid in cellular flues built as cellular walls. The most complete mode, however, is to have parallel ranges of steam-pipes of small diameter, communicating laterally by cocks. Then, when least heat is wanted, let the steam circulate through one range of pipes only; when more, open the cocks which communicate with the second range; and when most, let all the ranges be filled with steam.