Page:Scientific Memoirs, Vol. 2 (1841).djvu/236

224 The exact computation of the places of these two poles, according to our elements, gives them as follows:

1. In 73° 35′ north lat., 264° 21′ long, east from Greenwich, the value of the total intensity being = 1·701 in the unity in common use.

2. In 72° 35′ south lat., 152° 30′ long., the total intensity = 2·253.

At the first of these points $$\frac$$ has its greatest value, = + 895·86; at the second its smallest value, = — 1030·24.

According to Captain James Ross's observation the north magnetic pole falls 3° 30′ to the south of its position according to our calculation, which gives at that place a direction of the magnetic force, differing 1° 12′ from observation, as may be seen in the table of comparisons. We must expect a considerably greater displacement of the position of the southern pole. At Hobart Town, which is the nearest station to this pole, calculation gives too low a dip by 3° 38′, as far as the observation can be depended upon. It seems probable, therefore, that the actual south magnetic pole is considerably north of the position given by our calculation, and that it may be looked for in about 66° lat., and 146° long.

The two points on the earth's surface where the horizontal force vanishes, and which are called magnetic poles, may, it is true, be allowed a certain significancy on account of their relation to the form of the phenomena of the horizontal force all over the earth; but we must be careful not to give them undue consideration. The chord which unites these two points has no significancy, and it would be a gross mistake to call it the magnetic axis of the earth. The only mode of giving a generally valid signification to the idea of the magnetic axis of a body is laid down in the 5th Article of the Intensitas Vis Magneticæ, where it is understood to mean the straight line in which the moment of the free magnetism contained in the body is a maximum. In order to determine both the position of the magnetic axis of the earth in this sense, and the moment of the earth's magnetism in relation to this same axis, we only require, as noticed in Art. 17, a knowledge of the members of the first order of $$V$$. According to our elements. Art. 26, $$P' = + 925\centerdot 782 \cos u + 89 \centerdot 024 \sin u$$