110 THE PRINCIPLES OF FLORICULTURE 



with hubs so that 2-inch pipe may be caulked into the 

 fittings with rust joints, and a variety of hubbed coil 

 fittings may now be found on the market that are not to 

 be duplicated in screw fittings. The use of such fittings 

 is attended with a considerable saving of labor and ex- 

 pense for tools. 



" The mains of a gravity hot-water system are generally 

 run parallel to each other in cellar and trenches. The 

 flow main and return main are of the same size of pipe. 

 The expansion tank should be placed as high as possible 

 above the radiation, and be connected to the return main 

 near the boiler or to the boiler itself, as low as possible, 

 by the expansion pipe. The capacity of the tank should 

 be about one-twenty-fourth of the aggregate capacity of 

 the coils, mains and boilers. The boiler should be located 

 in a pit which it is always an advantage to have as deep 

 as practicable. The lower the heater below the radiation, 

 the more positive will be the circulation through the 

 pipes." 



84. Closed tank or pressure system of hot-water, 

 safety-valve control. - " With the closed tank system, 

 which is very little used now, excepting that there is a 

 growing use of the mercury seal, the coils of pipe are 

 arranged in the same way, whether of large or small pipe, 

 as in the open tank system. The mains are practically 

 of the same size, and the boiler is the same, the expansion 

 tank is provided with a weighted safety-valve at the vent 

 or overflow. This valve opens when the water reaches a 

 certain pressure. There is always some danger that the 

 safety-valve will become corroded and so become inopera- 

 tive, in which case an explosion is certain. The object 

 of the safety-valve is to put the water under a pressure 

 and so attain a higher temperature in the pipes, and 



