M. Quetelet on Atmospheric Electricity. 2i9 



vanished completely. Another remarkable fact observed by 

 M. Dove was, that among the numbers to whom he showed, in 

 bright daylight, the stereoscopic relief with blue and red edges, 

 one declared that he saw only the drawing with blue lines, as 

 through the red glass he could see nothing whatever. The eyes 

 of this individual in bright daylight were in the same condition 

 as a pair of normal eyes by twilight. 



XXXVIII. On Atmosphei'ic Electricity, according to the Obser- 

 vations at Munich and Brussels. Letter of M. Quetelet to 

 M. La MONT, Director of the Observatory at Munich^. 



I HAVE long upbraided myself for not having replied to your 

 obliging letter, wherein you requested me to make compa- 

 rative obsei-vations on the electricity of the air. ]\Iy purpose 

 was to request you, in the first place, to give me some instruc- 

 tions relative to the instruments which you have made use of, 

 and to the results at which you have arrived, so as to assure 

 myself that our observations might be compared with each other. 

 I have been partially able to satisfy my desire in this respect by 

 reading your description of the instruments used at Slunich 

 which you have been kind enough to send me, as well as the 

 article inserted in the 4th Number of Poggendoi-ff's Annalen 

 for 1852. 



In running over the table of your observations from 1850 to 

 1851, I have been struck with the small resemblance which sub- 

 sists between your numbers and those obtained at Brussels : to 

 enable you to judge of this, I will set side by side the monthly 

 results which you give for the hour of noon, and those which I 

 have obtained myself for the same hour. Your results are con- 

 tained in the second column, a, of the following table ; mine are 

 contained in the third column, b. You have seen from my first 

 investigation, published in the month of June 1849, that the 

 numbers immediately observed by the electrometer of Peltier do 

 not express the absolute values of the electric tension which are 

 given in the following column, b', according to each day's reduced 

 observations ; hence the last numbers are those which ought to 

 be compared with yours. In order to facilitate the comparison, 

 I have reduced all the values to the same unit, to the monthly 

 mean deduced from the results of the twelve last months which 

 occur in the table in the columns «, /S, and /3'. 



* From vol. ix. of the Bulletins de VAcad^nie lioi/ale de Bdyique. 

 Communicated by the Author. 



