HEATING THE RANGE 111 



perhaps a little better circulation, and also to diminish 

 the quantity of radiation necessary. This system is 

 not recommended for greenhouse heating. The desired 

 results are seldom attained in practice, and the element of 

 danger is too great." 



Mercury seal control. " The mercury seal, or heat 

 generator, as the manufacturers call it, is a device placed 

 on the expansion pipe between the boiler and the tank in 

 such a manner that the increased volume, or expanded 

 water must force itself through a pot of mercury, the 

 specific gravity of which is 13.6, which keeps all the 

 water between the boiler and the seal at a pressure of 

 about 13 pounds. This device is safer than a weighted 

 valve, but the same purpose may be accomplished by in- 

 creasing the elevation of the expansion tank, which is 

 certainly simple, although it is not always possible. It 

 might be added here that simplicity is undoubtedly a 

 virtue in greenhouse heating. Every valve and every 

 device of every kind should be as nearly fool proof as 

 possible." 



85. Forced circulation of hot-water. " There are 

 so many different combinations of forced circulation 

 units that it would be impossible to explain them all. 

 The simplest of these systems would be more correctly 

 called accelerated hot-water circulation. The system 

 is in every detail constructed like an open tank gravity 

 system excepting that a propeller, driven by electricity, 

 is inserted into one of the mains, and the propeller or 

 circulator increases the velocity of the circulation. This 

 device is practicable and of much merit. 



" It will be recognized that it is a difficult matter in 

 a large greenhouse range, heated by a gravity system of 

 hot-water, to make every coil an efficient unit. There 



