No. 4.] GREENHOUSES. 391 



for the crop grown in the coldest weather, for nothing 

 checks plants that are growing ra})idly so quickly as to have 

 them chilled ; and even plants that naturally grow at a low 

 temperature may be seriously injured if suddenly chilled 

 after a period of very rapid growth at a high tempera- 

 ture. 



Steam and hot water are about equally used in greenhouse 

 heating, the former being perhaps more largely used in heat- 

 ing large plants, while hot water is more used for small 

 houses. The main advantage of steam is that high heat can 

 be gotten up more quickly with it than with hot water, and 

 much less heating surface of pipe is required than with hot 

 water, thus making the fu'st cost of steam equipment a little 

 less than that of hot water. The advantages of hot water 

 are that a small amount of heat may be produced during the 

 fall or spring of the year, and heat Avill be secured more 

 cheaply. With the modern method of using small pipes for 

 distribution and radiation, both steam and hot Avater heat 

 can be distributed with equal rapidity after the fire is 

 started ; but perhaps steam may be carried a greater dis- 

 tance than hot water, unless the latter be under pressure. 



Boilers. 



The number of diifereut kinds of both steam and hot- 

 water boilers that are now in use is very great, and many 

 of them have reached great perfection ; but it is difficult to 

 prove that any one of the best of these is better than the 

 others, and one must visit some of the most successful 

 houses, and learn what are considered the best. 



A boiler to be satisfactory must have large grate area, 

 large heating surface in contact with the fire, and as much 

 water surface as is possible directly over the fire, as it is 

 here that the heat is the greatest and the water will be 

 moved the most rapidly. 



Most of the modern boilers are made in sections, so that 

 the size may be varied by the number of sections used ; and 

 if one section breaks, it can be replaced by a new one at but 

 little cost. Cast iron is largely used for greenhouse heaters, 

 both for steam and hot water, though for large plants the 



