OJS MR. T. HOPKINS ON 



case when they present large surfaces, from which the heat 

 can pass freely. Any thing that tends to compress or 

 condense the substance into a more compact body, injures 

 ■its radiating power. Thus a loose fleece of wool, if com- 

 pressed into a comparatively solid mass, will not radiate 

 equally well ; whilst polishing a piece of metal will deprive 

 it of a portion of its radiating power. 



Why there should be this difference in the radiating 

 powers of substances we do not know. There seems to be 

 some relation between the conducting and radiating pro- 

 perties of various bodies, the best conductors being the 

 worst radiators. The extent of surface also appears to have 

 considerable effect, as the greater the surface the greater the 

 radiation; hence the large amount of radiation from leaves 

 of trees and grass. 



Counter radiation has, however, wherever it takes place, 

 its full degree of effect in producing the general result. The 

 under sides of the upper fibres of loose Wool and leaves of 

 trees, like a cloud above the earth, will radiate heat down- 

 wards, and to a proportionate extent counteract upward 

 radiation from lower objects. And any contiguous lateral 

 substance will have a similar effect, as it will reciprocate the 

 radiation. A covering of the slightest kind may thus 

 counterbalance the upward radiation. 



Radiation of heat is, then, the cause of that cooling of 

 the surface of the earth during the night which takes place 

 under a clear sky, and that produces liquid dew on the earth: 

 but the cooling thus produced is not equally great in all parts 

 under apparently similar circumstances. It is the greatest 

 in the interior of large continents, and more particularly 

 where there is a very dry atmosphere, as in parts of Bussia, 

 the Desert of Bokhara, the great Desert of Northern Africa, 

 and other similar parts. Accounts of travellers in such places 

 represent the cold produced in them by radiation during 

 idear nights, as being more intense than in other countries 



