REFERENCE TO HORTICULTURE. 31 



greatest power is put forth in this country in June, while the greatest 

 temperature of the air does not take place till July." 



The construction of houses for growing the plants of warm climates, 

 or for forcing, is founded chiefly on the doctrine of radiation, as well 

 as on that of producing heat by combustion or fermentation. The 

 roof and sides of a frame or a hothouse serve the purpose of reflecting 

 back the heat of the bodies within, whether that heat is only such as 

 the soil enclosed naturally affords, or whether it is generated artificially . 

 But though the roofs of hothouses reflect back great part of the heat which 

 is radiated to them, yet a great part also is conducted through the glass 

 to its outer surface, and thence radiated into the free air. To prevent 

 this waste of heat, without diminishing the quantity of light transmitted 

 through the glass, is a desideratum in hothouse building. In Russia, 

 double sashes are used, and might be more generally adopted in this 

 country. A thin opaque screen of bunting or matting thrown over 

 the transparent face of the glass would likewise reduce the loss by 

 radiation to a minimum. 



The power of man over the heat of the free atmosphere is compara- 

 tively limited. Nevertheless, as heat is carried off from the surface 

 of the ground, and from all other objects, by wind, by radiation, and 

 by evaporation, it follows that heat may be saved by shelter from the 

 wind, and from being radiated into the air by a partial covering of the 

 ground, on a large scale, by scattered standard trees, or, on a smaller 

 scale, by covering beds or borders with straw ; and it may be saved 

 from being carried off by evaporation by under-draining, surface- 

 draining, and by such a composition of the soil as will readily admit the 

 infiltration of water, so as to render it at all times, except during 

 rains, tolerably dry. The drier the earth the less water will be evapo- 

 rated from its surface. The evaporation of water simply means its 

 being lifted up by the agency of heat or wind. For every drop removed' 

 a certain measure of warmth is lost. 



It takes above 200 of heat to convert water into steam ; and though 

 vapour does not require so much, part of the vapour being chemically 

 attracted by the atmosphere, still the consumption is great. From 

 these causes the earth and plants by degrees get so cold, from having 

 parted with their heat, that their temperature descends below the 

 freezing point. In spring and autumn the air is comparatively warm, 

 and the nights not so long ; and hence spring and autumn frosts seldom 

 take place till near sunrise : and if a cloud happens to settle above any 

 portion of the earth about that time, before the earth has been cooled 

 down to the freezing point, it prevents the farther radiation of the heat 

 upwards ; and hence we often find places lying contiguous and below 

 the cloud to be saved from frost at one time, while at another they 

 will be much hurt. Where plants partially cover one another, they 

 help to prevent radiation ; and when one plant is more covered with 

 moisture than another, more full of watery sap, and the bark more 

 tender, from these and other causes it is often, to all appearance un- 

 accountably, killed, while another is left unhurt. 



In order to protect plants from frost, we should study to have the 



