18 M. MELLONI ON THE FREE TRANSMISSION 



candle be held for some minutes at a suitable distance, and the com- 

 munication then intercepted, the needle will be forced back to zero in 

 an interval of time less than 8^. These operations would be impossible 

 if the side of the pile opposite to the lamp were hermetically closed. 

 The second moveable screen serves then to abridge the duration of the 

 experiments. It is particularly useful when the calorific action has 

 been very jDowerful or considerably prolonged, which sometimes happens 

 in the first attempts at adjustment. During these, the portions of heat 

 penetrate the pile to a great depth, and cannot return until a considerable 

 time hcis elapsed. Before these simple means of correction had occurred 

 to me, the difficulty of restoring the equilibrium of the two extremes of 

 the pile, as well as that which I experienced in respect to he different 

 temperatures of the screens and the apparatus, often obliged me to stand 

 still for fifteen or twenty minutes between two consecutive experiments. 



When any object of research requires numerous experiments, we 

 should endeavour from the very outset to avail ourselves of all that 

 contributes to make them more expeditious ; for the least delay arising 

 from imperfectness of method will, by gradually accumulating, ulti- 

 mately render the labour of whole days utterly fruitless. Yet, the at- 

 tention being absorbed by the main object, these little defects are at 

 first unnoticed. At length, however, we become sensible of them, and 

 endeavour to apply a remedy when it is almost too late. But the result 

 of the experiment is not without its use, since it may be more or less 

 serviceable in analogous circumstances. This consideration nmst be 

 my apology for the minuteness of detail into which I have entered. 



The first problem that presents itself, in the series of questions rela- 

 tive to the passage of radiant heat through solid bodies, is to determine 

 the influence which the degree of their polish has, and the quantity of 

 rays transmitted. In order to solve this, we have but to apply our 

 thermometrical method to several screens perfectly similar in all re- 

 spects, except as to the state of the surface. 



Out of the glass of a mirror which was very pure, and nine milli- 

 metres in thickness, I cut eight pieces sufficiently large to cover the 

 central aperture of the screen when they were placed on the stand ; and, 

 after having removed the quicksilver, I wore them down with sand, 

 emery, and other such substances, so as to form by their succession a 

 complete series of plane surfaces more or less finely wrought, from the 

 first and coai-sest to the highest and most perfect polish. These dif- 

 ferent pieces reduced to one common thickness of S^^'STl * and ex- 



* All the measures of small degrees of thickness contained in this Memoir 

 have been taken with a pair of calipers with pivots, a species of double com- 

 passes, with a spring and with legs of unequal lengths, much used in the manu- 

 facture of clockwork. This instrument measures directly, and with great nicety, 

 even the fortieth part of a line. 



