MELLONI ON THE NOCTURNAL COOLING OF BODIES. 459 



sible accuracy, experiments of comparison between the radiation 

 of lamp-black and of metals exposed to the nocturnal influence 

 of a serene sky. 



Those who have had occasion to compare with much accuracy 

 the movement of several thermometers placed in the same cir- 

 cumstances, will no doubt have been convinced that, whatever 

 be the skill of the maker, or the nature of the methods employed 

 to determine the points of comparison, it is with difficulty that 

 an identity can be obtained in the indications of two thermome- 

 ters. This defect would be of very little importance in researches 

 which, like ours, have for their object the detei'mination of merely 

 relative values ; for we might determine the differences existing 

 between them at a given temperature, and thus render the ob- 

 servations identical by a simple addition or subtraction. But 

 experience shows that the difference between the indications of 

 two thermometers does not generally remain invariable between 

 points of the scale at a distance from each other, and that it is 

 frequently variable within points which are near each other, ac- 

 cording as the two instruments are subjected to a more or less 

 abrupt variation of temperature. 



To overcome these difficulties, I in the first place chose from 

 my collection the three atmospheric thermometers which agreed 

 best with each other, and after having armed them in the manner 

 previously mentioned, I introduced them into their closed conical 

 recipients, and exposed them on the 9th of October*, at 8 o'clock 

 in the evening, the weather being very calm and serene. For 

 greater distinctness, the three thermometers are denoted by the 

 letters A, B, C. Half an hour afterwards we began to observe the 

 instruments, the indications of which, noted every three minutes, 

 gave the following results : — 



• On seeing an interval of twenty-two days between these experiments and 

 tlie preceding, it must not be inferred that in all the intermediate nights the 

 weather was not favourable for observations, as any one may convince him- 

 self by the successive dates ; but rather that the experiments in question did 

 not follow each other in the order adopted in this memoir. The ideas and facts 

 have not always proceeded with all the regularity desirable. Nevertheless the 

 work being finished, I have endeavoured to arrange the materials gutlicied in 

 the best way that I could. 



