30 



THE GARDENER. 



A moderately good contriver can do a great deal with 

 very inexpensive apparatus. 



A covering of flimsy material may appear very use- 

 less, but it is really most effective, and that it should 

 be so is explained on the principle of the radiation of 

 heat. 



Without entering into all the philosophy of this, as 

 clearly demonstrated by the late Professor Daniell, it is 

 enough to state, that any shading substance interposed 

 between the sun and a plant, or between the sun and 

 a bed of earth, intercepts the heat, which radiates from 

 it and sends it back again. A transparent covering, 

 such as paper or calico, allows the sun's rays to pass 

 through it, and yet, though very porous in texture, it 

 tends greatly to give back the heat which arises from 

 the earth, and which would otherwise be dissipated. 



The increase of warmth which may be imparted to 

 plants in winter by a coverlid of this kind is consider- 

 able. Even the branches of a tree overhanging a field 

 will, in a bright starlight night, preserve the ground 

 beneath, by radiating at a much higher temperature 

 than where there is no such awning. If the branches 

 could be brought nearer the ground, the temperature 

 would be raised accordingly. " 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 con- 

 tinually 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 



