88 



nearly at right angles during the morning and even- 

 ing hours, and the greatest amount of reflection 

 is secured during the middle of the day when there 

 is likely to be an excessive amount of heat. 



Two methods of heating are in common use — ^hot 

 water and steam. Each method has its advantages 

 and disadvantages. The first cost of installation 

 is considerably higher In the case of hot water, but 

 the cost of maintenance is usually less. Hot water 

 pipes retain their heat for a much longer time, but 

 on the other hand heat up much more slowly than 

 the steam pipes. In case of serious trouble with the 

 heater the hot water in the pipes may be able to 

 sustain sufficient heat to protect the plants until 

 repairs are made. Steam pipes cool off almost 

 immediately when the formation of steam ceases 

 and serious damage may result in case of a break- 

 down. 



The nature of the heating will depend to a con- 

 siderable extent upon whether the pipes are to be 

 placed overhead or under the benches. Hot water 

 pipes must be very much larger than those used for 

 steam and when placed above the growing plants 

 cut off a large amount of light, and on account of 

 their additional weight render heavier construction 

 necessary. Steam can be satisfactorily carried much 

 longer distances than water, but requires more 

 constant care. The first cost is less, however, and 

 repairs are less troublesome to make. 



The size of the pipe will, of course, vary with the 

 lengths of the run and the amount of radiation re- 

 quired. The minimum-sized pipes commonly used 

 with hot water are ij4 inches in diameter and with 

 steam three-quarter inch. The ratio of size of pipe 

 to radiation and glass surface has all been carefully 



