42 VEGETABLE FORCING 
this problem that a greenhouse heating specialist should 
be consulted before a decision upon any given amount of 
radiation is made, unless the matter has been determined 
by actual experience or observation. 
Systems of hot water heating.—There are three distinct 
forms of hot water heating, viz., the open tank gravity 
system, the pressure system and the forced circulation 
system. The gravity system is the oldest and is still used 
quite extensively. It provides for open tanks and large 
pipes. With it there must be ample radiation. Although 
plants thrive with this system, it is not popular with large 
commercial growers because of the excessive cost of 
installation, nor is it satisfactory in very long houses. 
The pressure system secured by the use of mercury and 
sometimes by safety valves is quite popular and satis- 
factory when properly installed and operated. Of the 
three methods of heating by hot water, the forced circula- 
tion system is the most satisfactory for large greenhouses. 
In this system the velocity of the circulation is increased 
by means of propellers or pumps operated by motors or 
engines. With forced circulation the mains and coils 
need not be so large as with the gravity system, so that 
the cost of installation is not greatly in excess of steam. 
This system does not require a large volume of water in 
the boiler and radiating pipes, so that the temperature of 
the house is under more perfect control than with the 
gravity system, and all parts of the house are heated 
uniformly, a condition not possible in large houses in 
which the gravity system is installed. 
Systems of steam heating —There are three systems of 
steam heating. (1) The low-pressure, steam-gravity re- 
turn. With this system the pipes are laid in the same 
general positions as with gravity hot water, care being 
taken to avoid water pockets. 
(2) Low-pressure steam with steam-return trap. It is 
often impracticable or undesirable to excavate boiler pits 
