74 ' HEAT, CONSIDERED WITH 



low situations the cold air, being heavier, collects, and not being benefited 

 by the dispersion of the wind and bringing forward a warmer air, plants are 

 much more liable to be hurt by slight frosts in such situations. Wherever 

 it is possible, when the clearness and coldness of the air indicate a tendency 

 to frost, plants that are worth the expense should be covered with the best 

 non-conducting substances we can fall in with. Metals are the worst, if 

 polished and bright in the colour : however, they are better non-conductors 

 than when dark-coloured and rough ; wood is still better ; but, unless when 

 saturated with moisture, woollen, next to furs and eider-down, is the best of 

 any, from the confined air retained between the hairs of the wool. 



230. Whatever covering is used, whether straw mats, bast mats, cloth, or 

 wool, or wood, they should be elevated above the surface to be covered, so as 

 to contain as much confined air as possible. Confined air is one of the worst 

 conductors of heat ; the covering will not radiate or give out heat till the 

 confined air and covering are both heated above the state of the atmosphere ; 

 and the transmission of heat will take place more slowly through the con- 

 fined air than anything else. Thus, for very little trouble, by elevating our 

 coverings, we surround our plants or plant-structures with a substance which 

 is very retentive of heat, and increases the power of the covering in an 

 immense degree. The heat has most tendency to ascend upwards, and this 

 should be most guarded against ; but it will also escape by the sides : and to 

 confine the air and heat completely, the plant or plant-structure must be 

 covered all round from the external air. A perfectly air-tight covering 

 would be with difficulty either procured or applied ; but apertures in direct 

 communication with the external air, may be guarded in such a manner as 

 to prevent the escape of heat. Thus, if we suppose four coverings of woollen 

 netting, with the meshes of 1-10 in. square open, and exactly as much space 

 between the meshes closed ; then these four covers would afford comparatively 

 little protection if placed so as the openings would be directly over each other ; 

 but by alternately placing over each other the open and the closed parts, the 

 egress of heated air, as well as the ingress of cold air, would be very much 

 interrupted. The warm air would have to deviate three times from its 

 direct upward tendency, which its greater elasticity, derived from the heat, 

 imparts to it ; and the cold air would have to turn as often from the course 

 in which, by gravitation, it would otherwise proceed downwards. The cur- 

 rents of both the internal and external air would thus be impeded, and the 

 interchange of temperature reduced to the very slow process resulting from 

 mere contact. 



237. Wall-trees should have a broad coping of wood on the wall, to pre- 

 vent the ascent of heat ; and woollen nets drawn down before tender peaches, 

 &c., in cold nights, and carefully removed in good weather through the day, 

 are a great help, when not left on in all weathers. The wall, for tender 

 fruit-trees, or other tender plants, is best built of porous materials, as bricks, 

 which retain the heat from the confined air better than stone ; and they 

 should be built with hollow chambers for the same purpose. Where paint- 

 ing is needed, white is the best colour. To prevent the bad effects of cold 

 east winds in the spring, causing the sap to descend in standard fruit-trees, 

 and destroying the blossom when expanded by the check it gives to the 

 ascent of the sap that should nourish it, the stems and branches should be 

 baund with straw ropes, and the ground mulched. 



238. Various situations should be chosen to protect tender shrubs and 



