248 M. Melloni on the Cause of the speedy Melting 



rays from the source of heat : hence it followed that the quan- 

 tity of melted snow is greater in the part shaded by the disk 

 than elsewhere, in spite of the smaller quantity of heat which 

 reached it. In the second case, again, where the source of heat, 

 and the pasteboard heated by its influence, furnish rays which 

 are nearly in an equal degree absorbable, the disk only dimi- 

 nished, by being interposed, the effect of the direct radiation, 

 and thus rendered the melting less in the shaded portion. 



From all this we conclude, that the speedy melting of snow 

 around plants, instead of being found in opposition to the exist- 

 ing theories of radiated heat, as M. Fusinieri alleges, is, on the 

 contrary, only a very simple consequence of it. 



It will probably be expedient to subjoin a few additional re- 

 marks, to account for the more minute details of this phenome- 

 non, — details which are easily explicable, when we start from 

 the principal fact, and take some accessory circumstances 

 into consideration. If, for example, it be demanded, why, be- 

 yond the power of the solar rays, the elevated temperature of 

 the air contributes to accelerate the speedier melting of the 

 snow around trees and solid bodies generally, standing in the 

 plain, the cause is easily found in the obstruction which these 

 bodies offer to the direct radiation from the snow towards the 

 celestial spaces ; this maintains them near the temperature of 

 fusion ; whilst the snow which is lying in exposed places is 

 cooled down many degrees below zero, in virtue of nocturnal 

 radiation, and is consequently much less disposed to melt un- 

 der the action of the ambient medium. With the same facility 

 we may explain why the influence of plants is still conspicuous 

 when the sky is entirely covered with clouds, and the tempera- 

 ture of the air below zero, for the diff'used heat of the sun pos- 

 sesses absolutely the same properties of transmission and ab- 

 sorption, as the direct heat, and ought, consequently, to pro- 

 duce effects wholly similar, so far as the intensity is concerned. 

 In considering the action of prolonged calorific radiation 

 upon a series of bodies endowed with the same absorbing 

 powers, it will be seen that those which possess a smaller mass 

 of matter should be heated more speedily than the others, and 

 arrive at that degree of heat which the state of the surface, the 

 power of the incident rays, and the pressure and temperature 



