On jSüght-Frosts. 9 



From this cirrnmstance follows a particular kinrl of niovements in the air. 

 The cooled air, being heavier, flows from the plants towards the groimd and, 

 along it, towards the lower parts of the field and, if there is no issue, it stays 

 there. As tliis movement lasts the whole night, the cooled layer of air on 

 the lowest places lises more and more, and the cooling is there much greater, 

 tlian on the places situated a little higher. 



A large trench can, as will be seen from some observations låter on, give 

 an issue to the greatest part of such a cooled layer. 



The above-named movement ought not to be mistaken for such movements 

 as are caused by a gentle breeze, however feeble it may be. The direction 

 of the particles of air in a wind always forms a little angle with the surface 

 of the ground, and hence results a warming effect, caused by the mixing of 

 the cold and warm layers of air and then by the heat, which the air conveys 

 to the ground, because, in conséquence of the oblique direction, new particles 

 always touch its siuiace. A horizontal movement will certainly be without 

 eftect, unless it sweeps away a thick layer of air. A breeze so feeble, that it 

 can scarcely move the leaves of the aspen tree, will on the contrary produce a 

 considérable warming. 



3:o. The conducting or radiating of heat from the ground to the 

 plants growing above it. 



In order to answer the question as to the effect of radiating heat from 

 the earth itself, during a summer night, we have only to considei' a pièce of 

 ground with plants. Let us look at the phenomena arising here and exercis- 

 ing a perceptible influence on the warming. 



From the fact that plants ladiate niore heat to space, than they re(;eive 

 from the ground, the latter becomes warmer than the plants and thus consti- 

 tutes a source of heat, the influence of which ought to be explained. 



The heat, which the ground lias received from the sun, pénétrâtes into it 

 and is conducted during the night towards the smface, radiating tlience to 

 the plants. 



I shall here confine the considération to a surface overgrown with plants 

 to an average thickness, by which the heat radiating directly to space can 

 be neglected. Différent kinds of soil are herein very dissimilar, depending on 

 the circumstance that the evaporation from that layer of earth, which is nearest 

 the surfare, is relatively great. 



