THE FORCING GARDEN. 



537 



quire no cleaning ; they may often pass under the paths, where 

 flues would extend too deeply. There is no danger of steam not 

 drawing or circulating freely, as is often the case with flues, 

 and always when they are too narrow, or too wide, or do not 

 ascend from the furnace to the chimney. Steam is impelled 

 from the boiler, and will proceed with equal rapidity along 

 small tubes or large ones, descending or ascending. Finally, 

 with steam, insects may be eflectually kept under in hot-houses 

 with the greatest ease, by merely keeping the atmosphere of 

 the house charged with vapour from the tubes for several hours 

 at a time. The boilers used to generate steam are formed of 

 cast or wrought iron, or copper, and of different shapes. 

 Wrought-iron, and an oblong shape, are generally preferred 

 at present, and the smoke-consuming furnace most approved is 

 that of Parke's. 



" The tubes used for conveying steam are formed of the 

 same metal as the boilers, but cast-iron is now generally 

 used. Earthen or stone-ware tubes have been ti'ied, but it is 

 extremely difficult to prevent the steam from escaping at their 

 junctions. The tubes are laid along or round 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 pro- 

 portion to its bulk, than smoke or heated air, steam-pipes are, 

 consequently, of much less capacity than smoke-flues, and 

 generally from three to six 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 good in circulating water or air, the mains may be of 

 a 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 dimension. This 

 is an important point in regard to appearance, as well as 

 economy. In order to produce a large mass of heated matter, 

 M*Fhail and others have purposed to place them in flues, 



3 z 



