THE STRUCTURE AND MANAGEMENT OF GREENHOUSES. 55 



lishments where no night fireman is employed and where conse- 

 quently the coal must be burned slowly with a dull draft, leaving 

 the fire to itself for several hours, there is little saving of coal by 

 using steam ; still the temperature can be more easily regulated 

 than with hot water, and the cost of fitting up for steam is less 

 than for hot water, since only about half as much pipe is required. 



The best arrangement of pipes for steam heating is to have one 

 flow pipe for each house passing overhead and branching at the 

 farther end into as many return pipes as are required. Each of 

 these return pipes is provided with valves at both ends and a 

 small vent cock near the lower end to blow off air. By this plan 

 the steam may be turned on to as many return pipes as are 

 required to keep up the temperature. One and a half inch pipe 

 answers for the flow, and one and a quarter inch or one inch for 

 the return. 



There is not the least trouble in maintaining circulation of 

 steam in all the pipes, provided the boiler is powerful enough to 

 keep up a pressure of two pounds to the inch, and provided that 

 the condensed water in the pipes drains back freely into the 

 boiler ; and for this purpose the steam pipes are best arranged at 

 a height of two or three feet above the water line in the boiler if 

 possible. Of course the friciion of the steam in passing through 

 a long range of pipes, together with the rapid condensation as it 

 passes along, tends to reduce the pressure, so that in a circuit of 

 five hundred feet of pipe 1 have noticed a difference in pressure 

 between the flow and return pipe near the boiler of from one to two 

 pounds. This would be balanced by from two to four feet perpen- 

 dicular height of water in the return pipe, and if there is not so 

 much difference of height between the return pipe and the water 

 level in the boiler, water will stand in the pipe, preventing circu- 

 lation and causing "•hammering" when the steam reaches it. 

 There are cases, however, where steam pipes must be used at a 

 lower level than the boiler, and then it becomes necessary to drain 

 them and return the water to the boiler by a trap or pump. 

 These are somewhat expensive and it is better to set the boiler 

 low enough to avoid the need of them. Where no watchman is 

 employed it is very necessary to use a boiler large enough to hold 

 steam for several hours without attention. This is easily done by 

 means of the automatic draft regulator, but the dull draft required 

 by this method is wasteful of coal though economical of care ; 



