MELLONI ON THE XOCTURXAL COOLING OF BODIES. 4G5 



The following are some of the results obtained by this method 

 on the night of the l/th of October 1846 :— 



In these experiments, as well as in every other measure re- 

 lating to the comparison of nocturnal cooling resulting from a 

 difference of emissive power, it is necessary to operate at a 

 distance from the ground and in dry weather ; for if the air is 

 very moist, and the aqueous vapour is precipitated by a slight 

 degree of cold, the differences marked by thermometers whose 

 armatures are coated, after shov. ing themselves at first, gradually 

 disappear. 



The reason of this fact, which has already been noticed in the 

 note to page 457, is easily perceived, if we reflect that in periods of 

 great humidity bodies of greater or less radiating power become 

 quickly covered with drops of water, and acquire upon the whole 

 the degree of emissive power belonging to this liquid. 



If the substances whose emissive power we M'ish to find can- 

 not be applied on the metallic armature of the thermometer, such 

 as different kinds of sand, earths, wood and leaves of plants, the 

 armature is then left polished, and the materials* are introduced 

 into the bottom of the conical recipient in such quantities that 

 when heaped up they nearly cover the reservoir. The cold 

 resulting from their radiation is propagated, as in the preceding 

 experiments, to the thermoscopic body, which eventually marks 

 a depression of temperature greater or less according to the sere- 

 nity of the sky, the tranquillity of the air, and the nature of the 

 radiating body. 



Seven vessels prepared in this manner, the first with lamp- 

 black in powder, the second with grass, the third with the leaves 

 of tlie elm and poplar, the fourth with vegetable earth, the fifth 

 with siliceous sand, the sixth with poplar sawdust, the seventh 



• "Metals" in the French tianslation — evidently a mistake. 

 VOL. V. PAKT XIX. 2 I 



