MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



(888.) 

 Variations 

 of mag- 

 netic ele- 

 ments. 



careful observations made in the Alps and Pyrenees 

 with Hansteen's apparatus, and studiously corrected 

 for temperature, the present writer has found a 

 diminution of one-thousandth part in the horizontal 

 intensity for a vertical ascent of about 3000 feet. 1 



Such are the general features of the distribution of 

 the magnetic force upon the surface of the globe. But 

 from an early period the magnetic elements for the 

 same place have been known to be variable. Such 

 deviations from constancy are either 1. Secular ; 2. 

 Periodic ; 3. Irregular. All the three magnetic ele- 

 ments (Variation, Dip, and Intensity) probably par- 

 take of these changes. The westerly Variation or De- 

 clination had till 1818 been increasing in Europe since 

 the earliest observations. This results from a com- 

 plicated movement of the line of No Variation and 

 its companion curves. While their common centre 

 or pole in North America has been slowly advancing 

 towards the East, a considerable portion of the sys- 

 tem of curves in this quarter of the globe has been 

 proceeding in a south-westerly direction nearly in a 

 line joining Spain with South America, thus produc- 

 ing a complicated rotatory motion of the lines of Va- 

 riation, of which the first effect was to extinguish the 

 singular loop in the curve of No Declination (in- 

 cluding a space of easterly declination) which is 

 shown in M. Hansteen's chart for 1600 to have 

 occupied a large part of Western Europe. 2 About 

 1818 the needle began to retrograde towards the 

 east in this part of Europe. The dip has been dimi- 

 nishing in Europe since the earliest observations. 



Graham discovered the Diurnal Variation of the 

 Compass. It occurs in a reversed direction in the 

 southern hemisphere. Near the equator (as at St 

 Helena) the needle partakes at one season of the 

 northern character, at another of the southern, de- 

 pending on the position of the sun. 



To Baron Humboldt and toArago we are prin- 

 cipally indebted for a knowledge of great and capri- 

 cious fluctuations of the magnetic elements occurring 

 simultaneously over vast regions of the earth. These 

 have been called magnetic storms. 3 They are often 

 connected with auroral appearances. 



(891.) Next to Professor Hansteen, science is mainly in- 

 debted for the recent great extension of our know- 

 ledge of the facts and laws of terrestrial magnetism 

 to two illustrious German philosophers, Baron Alex- 

 ander von Humboldt and the late Professor Gauss. 



(892.) rphe name of ALEXANDER VON HUMBOLDT is as 



widely known as science is cultivated. The ardour Baron 

 of his love of nature, the comprehensive interest which ^ x ^^ 

 he takes in every department of knowledge, and the toiat. 

 generosity of his disposition, in combination with the 

 fortunate accidents of an unusually vigorous constitu- 

 tion, and an eminent social position, have combined 

 to place him in the foremost rank of natural philoso- 

 phers. 



He was born on the 14th September 1769, and (893.) 

 consequently is now (1856) in his 87th year. He HU early 

 was a pupil of Werner in 1791, and devoted consi- 

 derable attention to metallurgy. An early longing 

 for foreign travel seems to have foreshadowed his 

 future career, but political circumstances were, to- 

 wards the close of the last century, eminently un- 

 favourable to its accomplishment. His celebrated 

 journey to Southern and Central America (under- 

 taken after the failure of several other schemes) lasted 

 from June 1799 until August 1804. We may, per- 

 haps, be allowed to regret that an impression of the 

 duty of presenting to the public the results of that 

 interesting journey in their most complete form 

 should have absorbed the leisure of so many of his 

 most vigorous years, and should havt withheld him 

 from other and still more important enterprizes. 

 From 1808 to 1826 Baron Humboldt resided mainly 

 in Paris, in the most intimate companionship with 

 Arago, who, though much his junior, has predeceased 

 him. He then took up his residence at Berlin, in 

 compliance with the wish of the King of Prussia, and 

 he soon after delivered there a course of public lec- 

 tures on Physical Geography which formed the basis 

 of his remarkable work entitled Kosmos, the compo- 

 sition of which has formed the chief occupation of his 

 vigorous old age. In 1829 he made a rapid but in- 

 teresting journey into Asiatic Russia, important in 

 its results, yet imperfectly carrying out the ardent 

 aspiration of his early years to unfold the marvellous 

 physical peculiarities of the Eastern continent. 



Baron Humboldt has contributed more to Physical ( 894 -) 

 ~ 1,1 ,-, T J j.i j. His studies 



Geography than any other man now living ; and that, in phy s i ca i 



not only by his individual efforts, but by the direction Geography, 

 and encouragement which he has given to innumer- 

 able travellers and naturalists. His career seems to 

 have been more closely modelled upon that of De 

 Saussure than of any other of his contemporaries or 

 predecessors. Those branches of Physical Geography 

 which admit of numerical treatment seem most con- 

 genial to him ; and he has left more of the impress 

 of his personal influence upon the sciences of Meteor- 



1 Edinburgh Transactions, vol. xiii. 



2 M. Arago gives, no doubt, an erroneous impression in stating that the westerly movement of the line of No Declination has 

 carried it in 200 years from Paris to Philadelphia. Yet I cannot subscribe to the opinion of his able commentator that the 

 Loop of No Declination which passed over Europe in the seventeenth century moved eastwards, and may still be traced in Asia 

 (British Association, Fifth Report, p. 63; Arago's Meteorological Essays, translated, p. 330). I conceive that, as stated in the 

 text, the loop moved south-westwards, became successively an oval and a singular point, and was finally worked out previously 

 to 1700. 



3 In September 1841 a remarkable simultaneous disturbance took place in Canada, Scotland, the Cape of Good Hope, and New 

 South Wales. Such occurrences are now known to be very common. 



