to the Antarctic Regions. 183 



lower magnetic latitudes than those of the total intensity, as its mi- 

 nima must in higher; and from such imperfect means as we have 

 of judging, the conjectural situations of the maxima may he stated 

 as occurring in 



20° N. 80° E. I. 



7 N. 260 E. II. 



3 S. 130 E. III. 



10 S. 180 E. IV. 



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 re- 

 moteness from those already known. Should any doubt arise, there- 

 fore, as to the greater or less eligibility of particular points, a refer- 

 ence 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 mag- 

 netic forces, — require, in the present state of our knowledge, the use 

 of those more refined instruments recently introduced. Being com- 

 parative rather than absolute, they depend in great measure (and 

 as regards the momentary changes, wholly) on combined and simul- 

 taneous 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 irre- 

 gular 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 return to their former values 

 in periods of very great and unknown magnitude. 



The recent discoveries connected with the irregular variations of 

 the magnetic declination, have given to this class of changes a pro- 

 minent interest. In the year 1818 M. Arago made, at the Obser- 

 vatory of Paris, a valuable and extensive series of observations on 

 the declination changes ; and M. KupfFer having about the same 

 time undertaken a similar research at Cazan, a comparison of the 

 results led to the discovery that the perturbations of the needle were 

 synchronous at the two places, although these places differed from 

 one another by more than, forty-seven degrees of longitude. This 

 seems to have been the first recognition of a phenomenon, which 

 now, in the hands of Gauss and those who are labouring with him, 

 appears likely to receive a full elucidation. 



