T E R 



237 



T E K 



of those occasional agitations in the needle, to which the 

 name oi' magnetic storms has been lately applied, and 

 which are now known to extend at the same moment over 

 a great portion of the earth's surface. 



The declination (variation) of the needle is that element 

 of terrestrial magnetism which was first observed, and the 

 difference of its amount in different regions, as well as the 

 annual chancre at the same station, was early noticed. Dr. 

 Halley, on his return to England after his second voyage, 

 during which he had made many observations on the varia- 

 tion in different parts of the Atlantic and Pacific oceans, 

 published, in 1701, a chart, on which were traced what have 

 been since called isogonal lines, that is, lines passing 

 through the points on the earth's surface where the variation 

 was the same : and other charts of a like kind have since at 

 different times been constructed. The expectation at first 

 entertained, that such a chart might serve as a means of 

 ascertaining the longitude of a ship at sea by an observed 

 variation of the needle has not however been fulfilled, 

 since as yet no formula has been discovered by which the 

 variation at any given time and place may with sufficient 

 accuracy be found : but though the changes of the varia- 

 tion have hitherto rendered such charts of little use for the 

 puqjoses of navigation, yet a knowledge of the form of the 



lines of equal variation at different periods may be of great 

 importance as a step to the discovery of the law of those 

 changes. The latest variation chart is one which was pub- 

 lished by Adolf Erman, after his journey, in company 

 with Hansteen and Due, through the "whole length 

 of the Russian empire, and his return to Europe by sea. 

 Erman determined the positions of the isogonal lines 

 from his own observations, and from the most authentic of 

 those which had been made by other observers between 

 the years 1827 and 1830 ; and subjoined is a representation 

 of the principal lines on the two hemispheres of the earth, 

 projected stereograph ically on the plane of the equator. 

 The lines marked o o pass through places where the varia- 

 tion is zero ; the positive sign before a number indicates 

 that the variation is westward, or that the needle deviates 

 to the west of the astronomical meridian ; and the nega- 

 tive sign indicates that the variation is eastward. On an 

 inspection of the lines, it is manifest that on a sphere they 

 must be curves of double curvature with bends in opposite 

 directions ; that most of them converge towards two points 

 on the earth's surface, one in or near Baffin's Bay, and the 

 other to the southward of New Holland ; and that be- 

 tween the inflexions there are some which return into 

 themselves. 



The dip, or inclination of the needle to the horizon, 

 which is another element of terrestrial magnetism, was first 

 recorded by Robert Norman [INCLINATION], and numerous 

 . ationshave been made to ascertain its value in dif- 

 ferent parts of the world, together with the variations to 

 which it is subject in process of time ; but a general chart 

 exhibiting the forms of the isoclinal lines, as those of 

 equal dip are called, is still wanting. On the above cut 

 an- represented by dotted curves some of the lines which 

 are best known ; and these have been taken chiefly from 

 the partial chart given by Major ("now Colonel ) Sabme, in 

 the ' Philosophical Transactions ' for 1840. The data are 

 stated to have been obtained from above 140 observations 

 made, on land between 1834 and 1839, and from many 

 which were made at sea by Mr. Dunlop in 1K51, and by 

 Lieut. Sulivan in 183!). Some points have also been taken 

 from the observations made by Erman in the Pacific 

 Ocean, of which la-st observations a table is given in the 

 'Seventh Ripoit of the British Association' (vol. vi.). 



". '/, /t, represents the line of no dip, which is evident I v 

 a curve of double curvature, and crosses the terrestrial 

 e<)ii.it(,r in two points at least : b, b, b, is the known portion 

 of the isoclinal line for a dip of 30 degrees below Hie 

 northern nart of the horizon : c, c, is the line for 60" ; and 

 d, rl, the line for 73. 



An inspection of the cut will show that the oval lines of 

 equal dip go on dimini-hing in magnitude northwards, and 

 the pole of the dip. or place where that element is a maxi- 



mum, may be fixed at a point (P in the diagram) in long. 

 263 (117 west), and in lat. 70 N. : at that place, by the 

 observations of Captain James Ross, the dip in 1831 was 

 found to be 89 59'. Professor Hansteen, of Christiania, 

 has deduced, from the observations which have been made 

 in the polar regions, that the isogonal lines in the northern 

 hemisphere tend to two points in the vicinity of the pole of 

 the dip ; those which are on the north side uniting in a 

 point a little way to the north of the latter pole, and those 

 on the south side a little way to the south of the same 

 pole. 



Till within the last fifty years it was the general opinion 

 that the intensity of terrestrial magnetism was the same at 

 all parts of the earth's surface ; and to the Academic cles 

 Sciences, in France, is due the honour of having been the 

 first learned body which proposed that observations should 

 be made for the purpose of determining that element. In 

 the instructions which its members drew up for the use of 

 the unfortunate La Perouse, it was recommended that the 

 intensity should be observed at places very distant from 

 one another, in order to ascertain whether or not any dif- 

 ferences existed in its value. The accounts of any observa- 

 tions which may have been made during the voyage pe- 

 riiihed ; but. between the years 1791 and 1794, M. Rossel, 

 who sailed from France with the expedition in search of La 

 Perouse, determined with a dipping-needle the inclination 

 to the horizon, and the times of performing a vibration, at 

 different places ; and from the latter the fact of a difference 



