534 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



towards the upper leaves, and consequently the temperature of 

 the last elementary stratum will, at first, be the highest of all. 

 But the air of the two upper strata will descend by virtue of its 

 greater specific gravity, and will at the same time react on the 

 radiating portions of the grass ; this reaction will be the more 

 enei'getic the more slowly the movement takes place. Now the 

 obstacles are less numerous in the first stratum than in the 

 second ; the air, therefore, will react more strongly in the latter 

 case ; and having thus caused a greater depression of tempera- 

 ture in the middle portion of the grass, it will itself participate 

 in this excess of cold through contact, and in its descent will 

 communicate it to the upper portion of the third subdivision, 

 which, again, will itself finally acquire a lower temperature to 

 that of the former. 



Thus the solid portions comprised in the three strata into 

 which we have supposed the grass of the meadow divided, com- 

 mence by a cooling proportioned to the quantity of heat which 

 each of them can vibrate freely towards space ; but the reaction 

 of the surrounding medium soon disturbs this order of things 

 to such an extent as to render coldest the leaves and stems, 

 •which are much less exposed to the aspect of the sky than the 

 summits of the herbage. The thermometer then ought to maintain 

 itself lower, when plunged to a certain depth in the grass, than 

 when placed in contact with the surface ; which is in accordance 

 with experiment*. This distribution of cold, and the greater 

 humidity which prevails in the midst of the grass, in consequence 

 of the evaporation from the ground, the transpiration of plants, 

 and the difficulty of renewing the air, will necessarily render the 

 precipitation of vapour more prompt in the interior than at the 

 surface of the meadow. But the descending motion of the air 

 continuing constantly in consequence of the cold due to the 

 upper portions of the grass, and the ascending motion in conse- 

 quence of the heat of the soil, if this is not too moist, its surface 

 will soon be dried. 



Then, the cold air which descends will itself become dried, 



* If the earth had no atmosphere, the minimum of temperature would 

 always he found in the parts most exposed to the aspect of the sky ; a thermo- 

 meter imbedded in the interior of the meadow would, at every hour of the 

 night, mark a higher temperature than that of a second thermometer placed 

 in contact with the summits of the grass. By this we see how much the pre- 

 sence of the air modifies the effects of nocturnal radiation, and how great has 

 been the error of neglecting the reaction of this fluid in the theory of dew. 



