32 HEAT, CONSIDERED WITH 



plants themselves and the earth around them as dry as possible to- 

 wards the evening. 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. Wherever it is possible, when the clear- 

 ness and coldness of the air indicate a tendency to frost, plants that are 

 worth the expense should be carefully covered with the best non- 

 conducting substances, such as straw, dry fern, mats, &c. 



Whatever covering is used, whether straw mats, bast mats, cloth, 

 wool, or wood, it 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 confined air than anything else. Thus, with 

 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 protection of the covering in an immense degree. In 

 doing so, however, the sides will have to be blocked up, or the heat 

 will rapidly escape at the vacuum between the cover and the glass 

 covered, both by radiation and conduction. 



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

 vent the ascent of heat ; and woollen nets or canvas, 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 painting 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 destroy- 

 ing 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 bound 

 with straw ropes, and the ground mulched. 



Various situations should be chosen to protect tender shrubs and 

 trees, according to the nature of the plant. For those that grow early, 

 and are apt to be nipped by spring frosts, a north border and cold soil 

 are best to retard their time of starting till the danger from frost is 

 less: for those that suffer from want of the wood being ripened suffi- 

 ciently, as many American plants which have a warmer summer in 

 their native situation to ripen the wood, as also for those that suffer 

 by autumn frosts before the wood is ripened, a south exposure, and 

 warm dry early soil are best : in dry soils there is not so much wood 

 made, but that which is made is more easily ripened ; and the more 

 sun, the more likelihood that the wood will be ripened before frost sets 

 in. In some late wet autumns, some of the hardiest of our trees have 

 been killed : transplanted Birch, after being some years transplanted ; 

 Oaks, that were apparently sound, dying down half their length in 

 the ensuing spring; and seedling American Oaks dying off in the en- 



