GLASS STRUCTUEES AND APPLIANCES. 



273 



as a supply cistern for filling and feeding the boiler. 

 Supply cisterns should never be connected with, the 

 flow-pipes, and where practicable they should be 

 made self-acting by having a high pressure ball- tap 

 affixed to a small supply pipe^ at a level that will 

 keep the cistern about three-fourths full, to admit of 

 expansion from the boiler before it overflows. The 

 expansion cistern and all pipes connected with it 

 should be placed where they will be quite safe from 

 frost. 



Air Pipes. — There are very few apparatus in 

 which the water would circulate unless some plan 

 were adopted for allowing the escape of air, which 

 invariably accumulates in the pipes. In large com- 



be let into the highest part of every set oi flow- 

 pipes. In some series, especially where there are dips 

 under doorways, or wherever there happens to be a 

 rise, more than one air vent may be needed ; for if 

 any air be allowed to concentrate, it will form as 

 much obstruction to the flow of water as a closed 

 valve. Air being 827 times lighter than water, it 

 cannot be made to descend under any amount of 

 pressure, consequently a small tap or pipe is the 

 only remedy. The diameter of these need not 

 exceed three-eighths of an inch. The pipes should 

 be secure from frost, and the points turned down as in 

 Fig. 47, to prevent any solid substance from getting in, 

 or the ejection of hot water over the foliage of plants 

 when the boiler is working under strong pressure. 



Fig. 47.— Feed and Air Pipes. 



plicated apparatus where pipes are fixed on diferent 

 levels, neglect of this precaution has been the cause 

 of many failures. 



Suppose we want the boiler (Fig. 47) to be filled by 

 the supply pipe at b, the pipe d will of course be filled 

 simultaneously with it. The water will also gradu- 

 ally rise in the boiler until it partially fills the upper 

 pipe c, and the air which is in the pipe x y will be 

 forced towards y by the weight of water behind it. If 

 the quantity of air is large enough, it will prevent 

 the junction at y, and cut off the communication. 

 If a small hole be drilled at y, the air will imme- 

 diately escape, when circulation will follow. In 

 houses where a very small lead pipe, e, can be carried 

 above the level of the supply cistern, an open vent 

 will be secured. In pits, small stop-taps must be 

 used, as pipes safe from frost cannot be carried high 

 enough. The taps should be turned occasionally, as 

 air is constantly accumulating, and every tap in the 

 largest apparatus should be open during the time 

 the boiler and pipes are filling with water. 



As more failures arise from neglect of this simple 

 arrangement than many people imagine, the ama- 

 teur engineer will do well to note that a vent should 

 66 



Pipes. — The pipes in general use for heating 

 hot-houses are three and four inches in diameter. 

 Pipes two inches in diameter are sometimes used, 

 but for all practical purposes those of the largest 

 size are to be preferred. The quantity of water 

 contained in a given length of each varies in pro- 

 portion to the square of its diameter. Equal lengths 

 of two, three, and four inches being taken, the 

 squares will be four, nine, and sixteen. Their 

 heating power is not, however, governed by the 

 water they contain, but by their radiating surface. 

 The three-inch contains half as much again as the 

 two-inch, and the four-inch twice as much as the 

 two-inch, and it is by these surfaces that all calcula- 

 tions are made. It was formerly, more than now, 

 the practice to use smaller pipes for connecting the 

 radiating pipes in the houses with the boiler, a pipe 

 one inch in diameter being sometimes employed to 

 supply a set of four-inch pipes. But pipes of 

 larger and uniform size answer best, particularly 

 where several hundred feet have to be heated from a 

 boiler which contains a great quantity of water. 



Quantity of Piping reqiiired. — The quantity 



