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Article XV. 



Memoir on the Nocturnal Cooling of Bodies exposed to a free 

 Atmosphere in calm and serene Weather, and on the resulting 

 Phcenomena near the Eaj'th's surface. {Second Memoir.) By 

 M. Melloxi*. 



[Read to the Roj'al Academy of Sciences of Naples on the 23rd of February, 

 and 9th and 16th of March 1847.] 



1 HE experiments described in the First Memoir (p. 453) 

 tended to prove — 



1 . That the emissive power of metals is much weaker than 

 has been hitherto supposed ; and that a thermometer contained 

 in a tin or copper case, exposed at night in the middle of the 

 fields, at a distance from substances which radiate heat strongly, 

 indicates very nearly the true temperature of the stratum of air 

 in which it is plunged, whatever be the state of the sky and the 

 calm of the atmosphere. 



2. That two thermometers, armed with their metallic cases, 

 one of which is polished and the other covered with lamp-black, 

 suspended in the free air by threads or tubes of metal, at the 

 same height, and during calm and clear weather, always mark 

 different temperatures, the blackened thermometer being con- 

 stantly lower than the poli&hed one, 



3. That the difference between the two radiations disappears 

 under the influence of a strong wind, or of a sky covered with 

 clouds, and is consequently the result of the unequal radiation 

 of the thermometers towards space, as has been admitted in 

 physics with reference to the nocturnal cooling of plants, since 

 the labours of Wells on the subject of dew. 



4. That the effect of the radiation of lamp-black is neverthe- 

 less greatly inferior to that which is generally attributed to vege- 

 table substances; for instead of 7° or 8°, it is 1°*5 or l°-7 in the 

 most favourable circumstances, which cannot be ascribed to an 

 inferiority in the emissive power of lamp-black as compared with 

 vegetables, but rather to the faulty method employed for deter- 



• Translated from t\\^ Annales de Chiniie et de Physique for April 1848,] 

 by the Rev. A. W. Hobson, M.A., St. John's College, Cambridge. The first 

 Memoir will be found at page 453 of the present volume. 



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