Intelligence and Miscellaneous Articles. 



79 



directly from its heated surface. These three kinds of rays are 

 combined in Mr. Piazzi Smyth's and Lord Rosse's experiments; I 

 proposed to myself to isolate them as much as possible, and to value 

 them individually, commencing with the luminous rays. 



In a preliminary experiment made by means of a differential air- 

 thermometer graduated by comparison with a very delicate mercurial 

 thermometer, I sought the first limit of the phenomenon I wished 

 to measure. The lunar rays concentrated on one of the bulbs 

 of the thermometer by means of an old lens of about 3 feet aperture 

 (belonging to the Imperial Observatory) produced no appreciable 

 effect on the instrument, each division of which corresponds to 

 o, 0043. Hence it was necessary to go much further. 



I had recourse to the thermopile*. An astatic galvanometer with 

 long needles, which I owe to the kindness of M. Ruhmkorff and of 

 the Ecole Normale, has enabled me to ascertain almost the hundred 

 thousandth of a degree : one division of the galvanometer is equal to 

 o, 00013 ; and the tenth of a division can be read off. 



The pile was placed behind the ocular of projection of an equatorial 

 of 9 inches, placed in the garden of the Imperial Observatory. The 

 distance from the pile to the ocular was such that the pencil of lunar 

 rays just covered the entire face of the pile. The apparatus, more- 

 over, was protected against external influences by a double metallic 

 envelope, and by four or five folds of black cloth. 



The telescope was pointed to the moon so that the image of 

 the satellite given by the seeker covered a circle previously traced 

 on a screen fixed to the apparatus ; and the regulator of the equato- 

 rial being set in motion, the object-glass was, during a definite num- 

 ber of seconds, alternately opened and shut by a stop, the least dis- 

 turbance in the direction of the telescope producing a displacement 

 of the needle. Each deflection observed when the stop was removed 

 or replaced was compared with the mean of the two observations im- 

 mediately before and after. Thus was avoided, as much as possible, 

 the influence of magnetic perturbations, which are always great under 

 an iron cupola. The following Table contains the mean of the 

 results of each series, opposite to the mean of the times of observa- 

 tion. The last column translates these deflections into decimals of a 

 thermometer degree : — 



Dates. 



Moon's age. 



Mean time. 



Mean 

 deflection. 



Value, in 

 degrees. 







h m 







Oct. 9, 1869. 



4th day. 



7 32 p.m. 



13 



000017 



10, „ 



5th „ 



7 46 



10, 



00013 



12, „ 



7th „ 



8 45 



58 



000075 



12, „ 



,» ,, 



9 12 



22 



000029 



17, „ 



12th „ 



8 39 



200 



0-00260 



20, „ 



15th „ 



10 11 



221 



0-00287 



* It was made with alloys of bismuth and antimony and of antimony 

 and cadmium, the preparation of which has been made known by M. E. 

 Becquerel, and which are endowed with a very high thermoelectric power. 



