Page:A Voyage of Discovery and Research in the Southern and Antarctic Regions Vol 1.djvu/46

xxxviii by any direct observation. They must of necessity however, lie in lower magnetic latitudes than those of the total intensity, as its minima must in higher, and from such imperfect means as we have of judging, the conjectural situations of the maxima may be stated as occurring in

Observations have been made of the horizontal intensity in the vicinities of II. and III., and are decidedly the highest which have been observed anywhere.

In general, in the choice of stations for determining the absolute values of the three magnetic elements, it should be borne in mind, that the value of each new station is directly proportional to its remoteness from those already known. Should any doubt arise, therefore, as to the greater or less eligibility of particular points, a reference to the existing magnetic maps and charts, by showing where the known points of observation are most sparingly distributed, will decide it.

For such magnetic determinations as those above contemplated, the instruments hitherto in ordinary use, with the addition of Mr. Fox's apparatus for the statical determination of the intensity, will suffice; the number of the sea observations compensating for their possible want of exactness. The determinations which belong to the second branch of our subject,—viz. those of the diurnal and other periodical variations, and of the momentary fluctuations of the magnetic forces,—require, in the present state of our knowledge, the use of those more refined instruments recently introduced. Being comparative rather than absolute, they depend in great measure (and as regards the momentary changes, wholly) on combined and simultaneous observation.

The variations to which the earth's magnetic force is subject, at a given place, may be classed under three heads, namely, 1. the irregular variations, or those which apparently observe no law; 2. the periodical variations, whose amount is a function of the hour of the day, or of the season of the year; and 3. the secular variations, which are either slowly progressive, or else