REFERENCE TO HORTICULTURE. 



71 



months, July and August, which are the only exceptions, a thermometer 

 covered with wool will sometimes fall to 35o. It is, however, only low 

 vegetation upon the ground which is exposed to the full rigour of this effect. 

 In such a situation, the air which is evolved by the process lies upon the sur- 

 face of the plants, and from its weight cannot make its escape ; but from the 

 foliage of a tree or shrub it glides off and settles upon the ground." 



227. " Anything which obstructs the free aspect of the sky arrests in 

 proportion the progress of this refrigeration, and the slightest covering of 

 cloth or matting annihilates it altogether. Trees trained upon a wall or 

 paling, or plants sown under their protection, are at once cut oflF from a 

 large portion of this evil, and are still further protected if within a moderate 

 distance of another opposing screen." {Ibid. vol. vi. p. 12.) 



228. Almost all the modes in practice of protecting plants are founded on 

 the doctrine of radiation, and hence the gardener should keep constantly 

 in his mind the fact, that all bodies placed in a medium colder than 

 themselves are continually giving out their heat in straight lines, and 

 that these straight lines, when the body is surrounded by air, may always 

 be reflected back on the body from which they emanate by the slightest 

 covering placed at a short distance from them; while, on the other 

 hand, if this slight covering is applied close to the body, instead of 

 reflecting back the heat, it will carry it off" by conduction : that is, the heat 

 will pass through the thin covering closely applied, and be radiated from 

 its surface. Hence, in covering sashes with mats, a great advantage is ob- 

 tained by laying straw between the mats and the glass, or by any other 

 means of keeping the mat a ffew inches above the frame. Hence also when 

 the branches of trees are to be protected by mats, they will be rendered 

 much more efficient if first surrounded by straw, fern, or some other light 

 body which contains in its interstices a good deal of air. It should be borne 

 in mind, Mr. Daniell observes, " that the radiation Is only transferred from 

 the tree to the mat, and the cold of the latter will be conducted to the former 

 in every point where it touches. Contact should therefore be prevented by 

 hoops or other means properly applied, and the stratum of air which is 

 enclosed will, by its low conducting power, eff^ectually 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." The 

 practice thus recommended in 1824 is now, 1841, generally adopted in the 

 management of plants on conservative walls. 



229. " The secondary effect which radiation has upon the climate of par- 

 ticular situations is a point which is less frequently considered than the 

 primary one which we have been investigating, but which requires perhaps 

 still more attention. The utmost concentration of cold can only take place 

 in a perfectly still atmosphere : a very slight motion of the air is sufficient 

 to disperse it. A low mist is often formed in meadows in particular situ- 

 ations, which is the consequence of the slow extension of this cold in the air, 

 as before described ; the agitation of merely walking through this conden- 

 sation is frequently sufficient to disperse and melt it. A valley surrounded 

 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 



