BY P. OLSSON-SEFFER, PH. D. 105 



increases with the height. From this circumstance follows a 

 particular series of movements in the air. The cooled air, by 

 reason of its increased density, flows from the plants towards the 

 ground, and slides down it towards the lower parts of the 

 field, and from the mountains to the valley, where it accumu- 

 lates, and if there is no issue it stays there. As this movement 

 lasts the whole night the chilled stratum of air on the lowest 

 places increases in depth, and the cooling is there much greater 

 than on the places situated a little higher. This movement, 

 which is a result of nocturnal radiation, ought not to be mistaken 

 for such movements as are caused by a breeze however gentle. 

 The direction of the particles of air in a wind always forms a 

 little anf le 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, owing to the oblique direction new particles 

 always touch its surface. A horizontal movement will certainly 

 be without effect, unless it sweeps away a thick layer of air. 

 A breeze so gentle that it will scarcely move the leaves of a tree 

 will produce a considerable increase of temperature. 



III. — The Radiating of Heat fbom the Ground to the 

 Plants growing on it. 



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

 heat from the earth itself during the night, we have only to 

 consider a piece of ground with plants. Let us look at the 

 phenomena arising here and exercising a perceptible influence 

 on the temperature. 



From the fact that plants radiate more heat into space than 

 they receive from the ground, the latter becomes warmer than 

 the plants, and thus constitutes a source of heat the influence of 

 which ought to be explained. ^ 



The heat which the ground has received from the sun pene- 

 trates into it, and is conducted during the night towards the 

 surface, radiating thence to the plants. Different kinds of soil 

 are in this respect very dissimilar, depending on the circumstance 

 that the evaporation from the surface layer of the earth is rela- 

 tively great. 



It is only in later times that attention has been directed to 

 this phenomenon by the researches of E. Russell, E. Wallny, 

 and S. Lemstrom. The latter has shown by actinometric experi- 

 ments that heat which radiates from the surface of the earth 

 after sunset is scarcely perceptible on a frosty night. 



K 



