JJOTTUiM HEAT 87 



The expression "ten degrees of Ijottom heat" means 

 that the bed should l)e tliat many degrees warmer tlian 

 the air just al)Ove the lied or in the propagating house. 

 Usually, however, the temperature of the bed is indicated, 

 as 60, 70, or some other degree. 



133. Natural bottom heat obtained from the sun during 

 the day is of importance at all times, but is scarcely con- 

 sidered in greenhouse winter work, except as something 

 to offset heat from the pipes in the propagating house. 

 In hotbed, cold frame and other structures not usually 



FIG. 75— MAKING A CONCRETE HOTBED 



The trenches being dug were later filled with concrete without "forms " 

 A.ter the concrete had "set" the earth was lalien out, partly bv horse scoop 

 partly by hand. ' 



heated artificially it is of great importance, especially in 

 spring. During the day the sun penetrates the surface 

 layer of soil, which becomes warm. During the night 

 this heat is given off slowly and checked by mats (Fig. 

 74)and shutters (Fig. 48), so the fullest use may be made 

 of it. The depth to which the soil may thus be warmed 

 depends upon the intensity of the light, the color and 

 other characters of the soil ; for instance, dark soils 

 absorb more rapidly than light ones. 



134. Application of bottom heat may be obtained in 

 any of the following ways: 



1. Hotbeds (Fig. 80) in which manure, tan bark, 



