OF TIIE MAGNETIC DECLINATION. 



13 



curves have another feature in common : the amplitude at its maximum value, 

 shortly after 6 A.M., is less than the amplitude at Toronto and Hobarton ; and, 

 upon the whole, the Philadelphia type confirms the idea that all forms partake of 

 the same general character, more or less affected by incidental irregularities. 



In reference to the annual variation, General Sabine, in the "rectifications and 

 additions" to the last volume of Humboldt's Cosmos, expresses himself as follows: 

 "Thus, in each hemisphere, the semi-annual deflections concur with those of the 

 mean annual variation for half the year, and consequently augment them, and oppose 

 and diminish them in the other half. At the magnetic equator, there is no mean 

 diurnal variation, but in each half year the alternate phases of the sun's annual in- 

 equality constitutes a diurnal variation, of which the range in each da}' is about 3' 

 or 4', taking place every day in the year except about the equinoxes ; the march of 

 this diurnal variation being from east in the forenoon to west in the afternoon, 

 when the sun has north declination, and the reverse when south declination." 

 According to the same authority, the annual variation is the same in both hemi- 

 spheres, the north end of the magnet being deflected to the east in the forenoon, 

 the sun having north declination; when in the diurnal variation, the north end of 

 the magnet at that time of the day is deflected to the east in the northern hemi- 

 sphere and to the west in the southern hemisphere. In other words, in regard to 

 direction, the law of the annual variation is the same, and that of the diurnal 

 variation the opposite, in passing from the northern to the southern magnetic 

 hemisphere. 



I next proceed to consider more in detail the annual variation at the hours of 

 6 and 7 in the morning and of 1 and 2 in the afternoon, these being the hours of 

 the principal and secondary maxima respectively. By subtracting the annual 

 mean from each monthly value at the respective hours, we obtain from the preced- 

 ing general table the following columns : — 



Annual Variation at the Hours op the Principal and Secondary Maxima of Range. 



t \ indicates j wes , t j deflection from the mean annual position. 



6 h ' A. M. 



7 h - A. M. 



Mean. 



January . 

 February . 

 March . 

 April . . 

 May . . 

 June 



July . . 

 August 

 September 

 October . 

 November 

 December 



+2'. 15 

 + 1.38 

 +0.75 

 —0.52 

 —1.32 

 —2.03 

 —1.72 

 —2.31 

 —1.14 

 +1.52 

 +1.28 

 +1.99 



+l'.8ti 

 +1.25 

 +0.19 

 +0.82 

 —1.43 

 —1.78 

 —2.16 

 —3.00 

 —1.34 

 +1.30 

 +1.66 

 +2.60 



+2'.01 

 +1.31 

 +0.47 

 +0.15 

 —1.38 

 —1.90 

 —1.94 

 —2.66 

 —1.24 

 +1.41 

 +1.47 

 +2.30 



Maximum range at the above hours, 

 5'.0 ; the easterly deflection being greater 

 by 0'.4 than the westerly. 



1&- p. M. 



2h. 



Mean. 



— 1'.20 



—1.09 



—0.19 



+0.88 



+0.92 



+0.84 



+1.21 



+1.39 



+1.24 



—1.24 



—1.31 



—1.43 



— 0'.76 



—1.15 



—0.75 



+1.25 



+0.94 



+0.93 



+1.30 



+1.23 



+0.40 



—1.05 



—1.40 



—0.96 



-ii-.ns 



—1.12 



—0.47 



+1.06 



+0.93 



+0.S9 



+1.25 



+1.31 



+0.82 



—1.14 



—1.35 



—1.20 



Range at the hours 1 and 2 P. M., 2'.7 ; 

 the eastern and western deflections being 

 equal. 



A general inspection of the above columns containing the mean values shows 

 that, approximately, the solstices are the turning epochs of this annual variation, 



