532 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



them will be cooled, will descend into the interior of the meadow, 

 and reach the ground. This movement of descent along the 

 leaves and stems will necessarily restore to it a portion of the 

 lost heat, and will force it to reascend towards the higher part of 

 the meadow, where it will undergo a fresh cooling, which will 

 cause a second descent, and so on ; so that, the air of the meadow, 

 or of our lower stratum, impelled by two opposite influences, 

 will soon take a circulatory motion, entirely analogous to that 

 observed in the water of a vessel placed on the fire. The cold 

 produced at the surface of the meadows will be gradually trans- 

 mitted, by this aerial circulation, to the lower parts, which will 

 also be cooled ; and, on the other hand, both by radiation and 

 by their contact with the superior portion of the stems, the tem- 

 perature of the whole mass of air which is put in motion in 

 the interior of the meadow, will fall. Suppose it sunk to 19°*5. 

 Now, according to the law which we have just referred to, the 

 grass ought to maintain itself constantly 1° below the surrounding 

 air : it will then have acquired half a degree of cold, and have 

 sunk from 19° to 18°-5. 



By repeating the same reasoning in these new conditions of 

 temperature, it is evident that the air will fall to 19° and the 

 grass to 18°. After that, the air arriving at 18°*5, the grass will 

 descend to 17°'5, and so on in succession ; so that by the action 

 of the grass on the air, and by the reaction of the air on the 

 grass, the temperature of the lower stratum will be gradually 

 lowered several degrees, and the space encumbered by the herb- 

 age of the meadow preserving all its vapour, will necessarily ap- 

 proach the state of saturation. Then, the thermometer introduced 

 into this space will mark a temperature much lower than that 

 of the higher stratum ; the hygrometer will there be kept near 

 its maximum of humidity, and the slightest degree of cold will 

 suffice to precipitate the aqueous vapour on the bodies which 

 are immersed therein. 



Before studying the distribution of the dew and of the cold 

 at diflferent depths of the meadow, let us remark, that the extra- 

 ordinary lowering of temperature presented in the preceding 

 experiments by the thermometers enveloped with cotton or wool, 

 compared with varnished or blackened thermometers (see the 

 first part of this memoir), is the result of an action entirely ana- 

 logous to that we have just been examining. In fact, the air, 

 cooled by contact with the higher portion of these envelopes. 



