470 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



substance will present analogous phaenomena, for example, grass 

 placed, as before described, on the bottom of the vessel. 



The comparison of what takes place with leaves of gold, silver 

 or copper, cut into strips of the same size as the grass, and 

 superposed in the same way round the metallic armature of the 

 thermometer, becomes then very interesting and decisive; for 

 in this latter case there is never either cold or dew; in the 

 former, on the contrary, the dew either does not show itself at 

 all, or is always deposited after the thermometer has indicated a 

 fall of some degrees below the temperature of the air. The in- 

 terval between the cooling indicated by the thermometer and the 

 precipitation of the dew is always very sensible even in the most 

 humid weather ; it frequently amounts to one or two hours when 

 the atmosphere has its mean degi-ee of moisture, and in periods 

 of great dryness the grass remains dry during the whole of the 

 night. We can also produce at will one or other of these phases, 

 and render longer or shorter the interval of time between the 

 indication of cooling by the thermometer and the appearance of 

 dew, by experimenting at greater or less elevations above the 

 level of the ground, on terraces and roofs of different heights 

 for instance ; for during calm and clear nights the moisture of 

 the atmosphere increases rapidly in approaching the earth's sur- 

 face, in consequence of certain actions and reactions of tempera- 

 ture between plants and the surrounding air, which we shall 

 presently examine. 



We have previously said that the experiments of Wells and 

 others do not give the value sought of the cold produced by 

 the radiation of a given substance, because the thermometers 

 used by these observers having their surface of glass and being 

 placed at different heights, we cannot deduce from their indica- 

 tions the true temperature of the stratum of air in which the 

 radiating body was plunged. 



In fact, our thermometers with metallic armatures in contact 

 with any substance being compared with a thermometer of the 

 same nature isolated in its conical vessel, have indicated to us 

 degrees of coolirg very much inferior to those resulting from the 

 experiments of Wells ; and nevertheless the recipients which 

 surrounded these thei'mometers increased by reflexion and by 

 the calm of the enclosed air, the radiation of the bodies towards 

 the sky. If these recipients be taken away, the differences of 

 temperature become still less, as results clearly from the follow- 

 ing series of observations : — 



