466 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



with mahogany 8a\Adust, furnished the following results on the 

 night of the 27th of September : — 



Name of radiating body. 



Temperatures 



Of the body. Of the air, 



Lamp-black , 



Different grasses with smooth leaves 



Leaves of elm and poplar , 



Poplar sawdust , 



Mahogany sawdust , 



Siliceous sand 



Vegetable earth 



17-50 

 17-24 

 17-17 

 17-51 

 1705 

 17-45 

 1702 



20-40 

 20-23 

 2010 

 20-38 

 19-80 

 2015 

 19-69 



290 

 2-99 

 2-93 

 2-87 

 2-75 

 2-70 

 2-67 



100 

 103 

 101 

 99 

 95 

 93 

 92 



The observations were made between 8 o'clock and 11-J. About midnight some 

 clouds appeared on the horizon on the side towards Naples; in a few minutes the 

 sky was completely covered. All the thermometers marked 20° nearly at 25 mi- 

 nutes past 12. 



Here the cold is rather less, because a part of the effect due to 

 the reflexion of the sides is no longer exerted on the thermo- 

 meter, as in the preceding case ; but the radiation and the con- 

 sequent cooling are manifested clearly in the sand, earths, wood 

 and leaves of plants, as well as in the lamp-black. And although a 

 rigorous comparison cannot be made between these results on 

 account of the differences of conducting power and mass of the 

 substances submitted to experiment, the emissive power of these 

 earths and vegetable substances did not seem to differ much 

 from that of lamp-black. 



Moreover, no one can doubt that the different degrees of cold 

 observed in these experiments arose from the emission of heat 

 into space or the elevated regions of the atmosphere by the difr 

 ferent substances which envelope the thermometric armature, 

 since they are seen to diminish at the very instant when clouds 

 come and station themselves like immense diaphragms above the 

 bodies, thus intercepting all communication between the sky 

 and the earth, and are at length completely reduced to zero. 



Another proof, not less conclusive, of the same truth, is the 

 cessation of the difference between the one and the other coated 

 thermometer, and the facility w ith which they all ascend to the 

 temperature of the thermometer whose armature is free, when 

 the openings of the recipients are closed by the metallic discs. 



As to the portion of the sky which acts with the greatest in- 

 tensity in this class of phaenomena, it is easy to convince oneself 

 that it consists in a certain circular space which has for its centre 

 the zenith of the place of observation. In fact, if, Mhilst experi- 



