164 GREENHOUSES 



few days and thus protect the plants from 

 injury by freezing, which would inevitably 

 result if only one boiler was in use. 



Heating Coils. Because of the large 

 amount of heating surface required, and be- 

 cause all parts of a greenhouse must be kept 

 at as nearly uniform temperature as possible, 

 radiators such as are used in private houses 

 have not been found practicable in green- 

 house heating. Instead, long coils of 

 wrought iron or steel pipe are used. For 

 steam heating these coils are commonly of 

 i or i ^4-inch pipe. In hot water heating they 

 are slightly larger, varying from i/4 to 2 

 inches. In the early days of hot water heat- 

 ing large cast-iron pipe, often as large as 

 four or five inches in diameter was used. It 

 is still used to some extent, but more often 

 in small private conservatories than in com- 

 mercial houses. 



There is very little to be said in favor of 

 using cast-iron pipes. The fact that they are 

 now so little used shows that they have no 

 special merit. The smaller, wrought pipe is 

 lighter, and much more easily handled; is 

 screwed together instead of caulked with 

 lead and oakum; has much more radiating 



