REFERENCE TO HORTICULTURE. 



73 



houses and botanic stoves in this country, where the plants Eire kept groAving' 

 throughout the winter, this mode of saving heat would, for many purposes, 

 exclude too much light. 



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

 tively Kmited. 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 from the wind by shelter, and in 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. Other modes of increasing the heat of 

 the atmosphere have been mentioned (231), or will readily occur; but per- 

 haps those of most practical value are shelter and adding to the dryness of 

 the soil. 



234. A distinction is to be made between increasing the heat of the atmo- 

 sphere and the soil^ and preventing the waste of the heat which they already 

 contain. This, also, is to be effected chiefly by counteracting radiation. Mr. 

 Lymburn, a scientific cultivator of great experience, has the following excel- 

 lent observations on this subject: — "The great effort," he says, " should be to 

 retain (if possible) the heat which was accumulated near the plants through 

 the day. If water be near, it has a tendency to assume the state of vapour, 

 and rob the air of its heat ; the sap of the plant may be more abundant, also, 

 from this cause, and increase the expansion of the fluids by frost, which may 

 end in the bursting and laceration of the vessels, and be the cause of death. 

 When a clear cold night succeeds to a wet day, if the night is long and the 

 atmosphere does not get cloudy, the heat radiates upwards from the earth 

 and plants into tiie cold air, while the evening at first is comparatively warm. 

 The cold is also greatly accelerated by the evaporation of moisture : it is 

 calculated that it takes above SOQo 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 

 anotlier 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, or growing more vigorously, more full of watery 

 sap, and the bark more tender, from these and other causes one plant is 

 often, to all appearance, unaccountably killed, while another is left unhurt. 



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

 plants themselves and the earth around as dry as possible towards the even- 

 ing. The situation for plants liable to be hurt by spring and autumn frosts 

 should be as much elevated as possible, in order to have the benefit of the 

 wind in dispersing the cold heavy air and bringing forward the warmer ; in 



