I 



Miscellanies. 385 



8. On the variation of the Magnetic Needle, by Prof. Bache. — 

 The observations were made at West Chester, twenty one miles to the 

 west of Philadelphia, during ten days in the latter part of August and 

 beginning of September 1832 ; they were continued generally during 

 the whole night as well as the day, the needle being observed every 

 hour except when circumstances connected with the phenomena 

 showed such attention to be unnecessary. The series of results, 

 though limited, is valuable on this, account. The actual westerly 

 variation, of the place of observation was 3° 25'. The occurrence 

 of two maxima and two minima of westerly variation, is shown dis- 

 tinctly, by the results of each day's observations, confirming for this 

 place of observation the results of Canton obtained at London, 

 when the variation was 19° W., the subsequent deductions of Gil- 

 pin, and the still later ones of Colonel Beaufoy obtained when the 

 variation was between 24'-' and 25'-' W. In regard to the hours of 

 occurrence of these maxima and minima, there is not the same 

 agreement. Mr. Christie gives from Canton's results the times of 

 maximum westerly variation, at half past one P. M. and near mid- 

 night, and the times of minimum, at nine A. M. and nine P. M., and 

 from Col. Beaufoy 's observations tlie first maximum and minimum 

 at times rather earlier, than those deduced from Canton's observa- 

 tions, and the second maximum about eleven P. M. During the 

 time embraced in the series by Prof. Bache, the first maximum of 

 westerly variation, occurred between two and half past four P. M. 

 and the second between eight and a quarter, P. M. and twelve and 

 two thirds,' A. M., varying with meteorological circumstances. The 

 two minima of westerly variation occurred, the first, between eight 

 and nine A. M., and the second between six and seven and a quar- 

 ter, P. M. The night maximum was fi-equently greater than the 

 day maximum. The observations are represented by broken lines 

 which enables the eye readily to trace the results. The mean times 

 of maximum variation west, are three P. M., and twelve midnight, 

 and of minimum, nine A. M. and six P. M. The mean midnight 

 maximum is four minutes and a quarter higher than the day maxi- 

 mum, and the morning minimum, one minute and a quarter less 

 than the evening minimum. 



The author concludes that ordinary meteorological phenomena, 

 such as the formation and dispersion of clouds, the occurrence of 

 showers and gusts, of fogs, Stc, have a powerful modifying effect 

 on the diurnal variation : in one remarkable case of a shower fol- 



VoL. XXVII.— No. 2. 49 



