252 M. Quetelet on Atmospheric Electricity. 



where F(0) represents a small correction dependent on the 

 angle 0. You consider that this correction, and that due to the 

 torsion of the fibre, may be neglected through the extent of an 

 arc of about 65°, which is represented by nine divisions of your 

 scale. 



This result of your calculation does not agree with the results 

 deduced from observation by M. Peltier and me, even for feeble 

 electric tensions. In admitting it, the values /3 and /S' of the 

 first table relative to Brussels would be sensibly equal, which is 

 far from being the case. This is an essential point, to which I 

 permit myself to direct your attention. 



Taking, with you, the values directly obsei'ved at Brussels as 

 repi'esenting the electric tensions of the air without apphjing any 

 coirection, I find that my numbers come very near to yours, and 

 that the ratio of summer to winter is less than that of 3 to 1 ; 

 but is this substitution legitimate ? 



Permit me to submit to you one other observation : you say 

 at the fifth page of the description of the new instruments and 

 apparatus at the Observatory of Munich, that the electrometer 

 which you have made use of is constructed after the principle of 

 the instrument of Peltier used at the Observatory of Brussels ; 

 but that the method pursued to determine the electric tensions 

 of the air from the readings of the instrument is essentially dif- 

 ferent. I find, in fact, in your description all the principal parts 

 of the electrometer which has served for my observations, and 

 which was constructed for our observatory by M. Peltier; I 

 remark, however, one important difference in the proportions : 

 the ball which surmounts my insti'ument is considerably larger 

 than yours, at least if I may infer from the drawing, for you 

 have given no dimensions. 



I could have desired to know the motives which have induced 

 an observer so skilful as yourself to reduce the ball to a dimen- 

 sion so small relatively to the stem which it surmounts ; it seems 

 to me that this reduction must have for its effect a reduction of 

 the sensibility of your apparatus. It is in this sense that M. Pel- 

 tier has remarked, that the induced electricity coerced at the 

 extremity of the stem leaves to that of the contrary name the 

 rest of the length whereon to distribute itself; but the longer 

 the stem is relatively, the less will be the portion of it which 

 returns to the indicating needle, and the less will be the diver- 

 gence. 



You will excuse me, my dear confrere, for thus submitting to 

 you my doubts. It appears to me of the greatest importance to 

 recognise the true cause of the errors, if such exist ; and I do it 

 with all the confidi uce with which your talents, and the love of 

 truth which animates both of us, inspire me. 



Brussels, August 5, 1852. 



