u \ of Terrestrial Magnetism, 395 



the west until 2 hours, which is the period of the extreme west- 

 ern limit/' (Toronto Observations, vol. i. p. 14.) And at Ho- 

 barton as follows : — " The north end of the magnet has two 

 eastern and two western elongations or turning-])oints, at both 

 periods of the year; from October to February the principal 

 eastern elongation is at 2^, and the minor one at 15^; from 

 April to August the hours of these turning-points become 

 respectively 3^ and 16^; from October to February the principal 

 western elongation is between 20^ and 21^, and the minor one 

 at ll^j whilst from April to August the corresponding pha3- 

 nomena occur at 22^ and ] 1^." Then, comparing the figures 

 which represent these movements with those of Toronto, he 

 concludes that they are identical, only having opposite signs, 

 except that the turning-points or periods are earlier at Toronto than 

 at Hobarton. 



The opposition of these movements is shown in our figure in 

 a manner easily to be remembered. The two stations may be 

 regarded as [within limits, Ed.] the type of all that happens out of 

 the torrid zone. Within or near the tropics the law holds good, 

 providing we have regard to the hemisphere in which the sun is, 

 the places being considered as in the southern hemisphere when 

 the sun is in the northern hemisphere, and in the northern hemi- 

 sphere when he is in the southern. If there should sometimes 

 appear to be an exception, it would be only an apparent one, as 

 we shall soon demonstrate. In the meantime, to facilitate com- 

 parisons, we may establish the following : — 



Corollary 1. — All the variations are the same in both hemi- 

 spheres, providing we change the name of the pole influenced ; 

 and if we take as the type the north pole and northern hemi- 

 sphere, we shall have identical variations for the south pole in 

 the southern hemisphere; and the variations of the north pole 

 in the southern hemisphere will be opposite to those in the 

 northern hemisphere. 



Remark. — Perhaps, to avoid any misunderstanding, and the 

 confusion of poles with hemispheres, and for greater convenience 

 in the indication of the antagonistic forces of poles in which the 

 pole called north is the true south pole of the needle, it might be 

 better to retain the name of marked pole, formerly used by some, 

 and especially by English writers, to designate the fundamental 

 pole, to which all is referred, and which in our part of the world 

 looks to the north. 



Corollary II. — As the points of inflexion of the diurnal curve 

 depend on the sun's passage of the magnetic meridian, it follows 

 that if two places in the northern hemisphere have opposite 

 magnetic declination, i. e. the one east and the other west de- 

 clination, the second will be in its phases later [Qu. earlier, Ed.] 



2D2 



