204 FIXED STRUCTURES FOR GROWING 



which requires no explanation. Where the circulating pipes are below the 

 level of the floor of the house, and where there is to be 

 a raised pit for containing plants, a tank or cistern might 

 be formed under it of the length and width of the pit, and 

 of a depth equal to the distance between the upper and 

 lower heating-pipes ; and with this tank the pipes might 



rig 145. Sedion of communicate by means of stop-cocks; so that whenever 



reserve cistern and there was more heat in the pipes than was wanted for heat- 



hot-water pipes. j n g ^he a j r O f the house, it could be transferred to the 



reservoir tank. To save the expense of stop-cocks where the cisterns could 



be wholly or partially uncovered, the orifices of the connecting pipes might 



be stopped by plugs ; and when the reservoir tank is above the level of the 



heating-pipes, the connexion between them might be made by means of 



syphons with stop-cocks. 



600. The pipes employed are generally of cast-iron, and round, as being 

 more conveniently cast ; but any other metal and form will answer ; and 

 when there is no great pressure on the pipes earthenware may be used, the 

 joints being made good with cement ; and at the angles, where elbow-joints 

 would be necessary, small cisterns could be employed, or elbows of earthen- 

 ware might be made on purpose. For obtaining a large heating surface, flat 

 cast-iron pipes have been used in some cases, placed vertically, and in 

 others horizontally ; but round pipes of four inches in diameter are in 

 most general use. When the object is to obtain a supply of heat in the 

 shortest time, then the boiler and pipes should be of small capacity; 

 and this is generally desirable in the case of greenhouses, where heat is 

 occasionally wanted for a few hours in damp weather, not for the sake of 

 raising the temperature, but for drying up cold damp : nevertheless, even in 

 greenhouses it is desirable to have a reservoir of heat for supplies in very 

 severe weather. In stoves in which fire-heat is employed the greater part 

 of the year, both boiler and pipes may be of large capacity ; and this should 

 also be the case in early forcing -houses. Whatever mode of heating or kind of 

 pipes may be adopted, the pipes should always have a gradual ascent from the 

 place where they enter the house, or are intended firstto give out heat, towards 

 the farther extremity; otherwise, the circulation will be less rapid, and conse- 

 quently the heat less equally distributed. The quantity of pipe required to 

 heat any house depends on various circumstances ; such as the form and con- 

 struction of the house, the temperature that is to be kept up in it, and the 

 temperature of the external air. Various calculations have been made on the 

 subject by different engineers, and more especially by Mr. Hood, who says, 

 " It may be taken as an invariable rule, that in no case should pipes of a 

 greater diameter than four inches be used, because, when they are of a larger 

 size than this, the quantity of water they contain is so considerable, that it 

 makes a great difference in the cost of fuel, in consequence of the increased 

 length of time it will require to heat them, which is four and a half hours 

 for four-inch pipes, three and a quarter hours for three-inch pipes, and two 

 and a quarter hours for two-inch pipes, supposing the water to be at 40 be- 

 fore lighting the fire, and the temperature to which the water was raised, 200. 

 Pipes of two or three inches diameter therefore are to be preferred from green- 

 houses, conservatories which only require fire-heat to be applied occasionally." 

 After calculating the loss of heat from exposed surfaces of glass under dif- 

 ferent circumstances and situations, Mr. Hood gives the following rules for 



