30 HEAT, CONSIDERED WITH 



by its low conducting power, effectually secure the plant. With their 

 foliage thus protected, and the roots well covered with litter, many 

 evergreens might doubtless be brought to survive the rigour of our 

 winters which are now confined to the greenhouse and conservatory." 



" A very slight motion of the air suffices to break or check the force of 

 radiation. The mere act of walking through a mist in a meadow has 

 often been found sufficient to dissolve and dissipate it. A valley sur- 

 rounded by low hills is more liable to the effects of radiation than the 

 tops and sides of the hills themselves; and it is a well-known fact that 

 dew and hoarfrost are always more abundant in the former than in the 

 latter situations. It is not meant to include in this observation places 

 surrounded by lofty and precipitous hills which obstruct the aspect of 

 the sky, for in such the contrary effect would be produced. Gentle 

 slopes, which break the undulations of the air without naturally circum- 

 scribing the heavens, are most efficient in promoting this action ; and 

 it is worthy of remark and consideration, that by walls and other 

 fences, we may artificially combine circumstances which may produce 

 the same injurious effect. 



" But the influence of hills upon the nightly temperature of the valleys 

 which they surround is not confined to this insulation ; radiation goes 

 on upon their declivities, and the air which is condensed by the cold, 

 rolls down and lodges at their feet. Their sides are thus protected 

 from the chill, and a double portion falls upon what many are apt to 

 consider the more sheltered situation. Experience amply confirms 

 these theoretical considerations. It is a very old remark, that the in- 

 jurious effect of cold occurs chiefly in hollow places, and that frosts are 

 less severe upon hills than in neighbouring plains. The leaves of the 

 Vine, the Walnut-tree, and the succulent shoots of Dahlias and Pota- 

 toes, are often destroyed by frost in sheltered valleys, on nights when 

 they are perfectly untouched upon the surrounding eminences ; and 

 the difference, on the same night, between two thermometers placed in 

 the two situations, in favour of the latter, has amounted to thirty 

 degrees. 



11 The horticulturist can effect but little in the way of raising the 

 temperature of the climate in the open air, except by the choice of a 

 sunny elevated situation, thorough drainage, the concentration of the 

 sun's rays upon walls, and furnishing shelter. The natural reverbe- 

 ration of heat from walls and the earth itself is, however, very great, and 

 generally effective in enabling the plants to resist the exhaustive force 

 of radiation. Few of the productions of the tropical regions are ex- 

 posed to a greater heat than a well-trained tree upon a wall in summer. 

 It has been proved by experiment that the power of radiation from the 

 sun, like that of radiation from the earth, increases with the distance 

 from the equator ; and there is a greater difference between a ther- 

 mometer placed in the shade and another in the solar rays in this 

 country, than in Sierra Leone or Jamaica. This energy of the sun is 

 at times so great, that it often becomes necessary to shade delicate 

 flowers from its influence ; and it would at times be desirable to try 

 the same precaution with the early blossoms of certain fruit trees. The 



