72 HEAT, CONSIDERED WITH 



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 con- 

 trary effect would be produced. Gentle slopes, which break the undulations 

 of the air without naturally circumscribing 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." 



2-30. u 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 

 adouble 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 injurious 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 Potatoes, 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." 



231. " Little is in the power of the Horticulturist to effect in the way of 

 exalting the powers of the climate in the open air ; except by choice of situa- 

 tion with regard to the sun, and the concentration of its rays upon. walls and 

 other screens. The natural reverberation from these and the subjacent soil 

 is, however, very effective, and few of the productions of the tropical regions 

 are exposed to a greater heat than a well-trained tree upon a wall in sum- 

 mer. Indeed, it would appear from experiment that the power of radiation 

 from the sun, like that of radiation from the earth, increases with the dis- 

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

 meter 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 influ- 

 ence ; and I have already pointed out (227) a case in which it would be 

 desirable to try the same precaution with the early blossom of certain fruit- 

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

 greatest temperature of the air does not take place till July. The tempera- 

 ture of summer may thus be anticipated a month in well-secured situations." 

 (Ibid. p. 16.) 



232. 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 hot- 

 houses 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 hot- 

 house building. In Russia double sashes are used, and while the plants 

 within are in a dormant state little injury is sustained by them ; but in green- 



