OF THE MAGNETIC FORCE. 



53 



delphia ; from Table LXVIII, Vol. Ill of the Toronto Observations, we find the 

 maximum at 5 h , a secondary minimum at 14 h , a secondary maximum at 18 b , and the 

 minimum at 22 h ; the maximum is therefore apparently delayed at Toronto 4 h , the 

 minimum 4^ h , the secondary wave is likewise retarded by about 4 hours. This 

 epochal difference I take, most likely, to be a distinctive feature due to the localities ; 

 there is also a remarkable difference in the amount of the diurnal range as will 

 presently appear. The degree of sensibility in the adjustment of the centre of 

 gravity of the instrument affects most the latter difference, whereas the epochal 

 difference may be supposed to depend, in a measure, upon the sensibility of the 

 magnet in regard to changes of temperature and consequent changes of magnetism. 

 The change in the adopted value of the correction for 1 of change in the tem- 

 perature (expressed in scale divisions) as used in present reduction (10.8), and as 

 used in four volumes of record and reduction (13.5) gives us the means of a partial 

 test of the effect on the epochs, we find from the plates in Vol. IV the time of the 

 maximum 1| P. M. and of the minimum 11| P. M., which though somewhat nearer 

 to the Toronto epochs, still leave a large discrepancy. 



The diurnal range at Toronto is very much less than at Philadelphia; in 1841-42 

 the range was but one-half of that observed at Philadelphia, and for later years (see 

 Table LXVIII of Vol. Ill of the Toronto Observations) the ranges compare as fol- 

 lows: Toronto 0.00019, Philadelphia 0.00071. 



In diagram G the diurnal range for each month is exhibited (expressed in parts 

 of the force). 



