OP THE MAGNETIC FORCE. 



53 



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

 maximum at 5^, a secondary minimum at 14'\ a secondary maximum at 18"", and the 

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

 minimum 4|'', 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 diflference m 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 redviction (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 1 P. M., which though somewhat nearer 

 to the Toronto epochs, still leave a large discrepancy. 



Table VIII. — Amplitude of the Diurnal Variation of the Vertical Force. 





Maximum 

 0.00 



Minimum 

 0.00 



Eange 

 0.00 



Maximum 

 0.00 



Minimum 

 0.00 



Range 

 0.0 



January 

 February 

 March 

 April . 

 May . 

 June . 

 July . 

 August 

 September 

 October 

 November 

 December 



t 











028 

 038 

 029 

 034 

 043 

 057 

 060 

 068 

 055 

 025 

 024 

 026 



031 

 035 

 022 

 039 

 042 

 054 

 057 

 051 

 041 

 029 

 013 

 026 



059 

 073 

 051 

 073 

 085 

 110 

 117 

 119 

 097 

 054 

 037 

 052 



359 

 484 

 377 

 431 

 555 

 725 

 7ri9 

 878 

 712 

 323 

 313 

 330 



398 

 447 

 283 

 500 

 535 

 686 

 733 

 654 

 632 

 369 

 166 

 332 



0757 

 0931 

 0660 

 0931 

 1090 

 1411 

 1502 

 1532 

 1244 

 0692 

 0479 

 0662 



Summer 

 Winter 

 Year . 













052 

 027 

 039 



046 

 025 

 032 



098 

 052 

 071 



667 

 349 

 503 



597 

 322 

 410 



1264 

 0671 

 0913 





In parts of the force. 



In absolute measure. 



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). 



