296 HISTORY OF DISCOVERY RELATIVE TO MAGNETISM. 



great sensation, and in order to verify it and to discover from observations the 

 law of the variation of the magnetic needle, Halley obtained, through the in- 

 fluence of King William, the command of a small vessel of the royal navy, iu 

 which he made two voyages in the yc-ars 1G98 and 1699. lie soon returned 

 from the first voyage on account of his crew having fallen sick after })assing 

 the equal or, and also on account of the mutiny of Lis lieutenant. In 1699 he 

 sailed again and cruised iu the Atlantic and Pacific oceans in various directions. 

 From these voyages he gathered a sufficient number of observations to enable 

 him to prepare his celebrated prospective chart of the variations of the mag- 

 netic needle. 



On this chart he counected with continued lines all the places on the earth 

 where similar and equal deviation of the needle had been observed, and thus 

 produced a projection of what is called the lines of equal variation, or isogenic 

 lines. These lines afford a ready means of presenting at once to the eye the 

 totalit}' of the phenomenon. They are also sometimes called Ilalley's lines, 

 although, as may be inferred from a passage in Kircher, he was not the first 

 who constructed such charts. Kircher, , in fact, states, at page 443 of his 

 Nautica Magnetica, that a Father Chr. Burrus had thought he had discovered 

 a process by which longitude at sea might be determined, and had on account 

 of it claimed a reward of 50,000 ducats from the King of Spain. His state- 

 ment is as follows : On his voyage to India he observed, under the widely dif- 

 ferent meridians, the deviation of the magnetic needle, and collected also obser- 

 vations made by others. These observations, the number of which was not 

 inconsiderable, he projected on a map, and then connected the places of equal 

 variation by lines, which he called chalyboclitic lines. He asserted confidently 

 that, by means of these lines, he could accurately determine the geographical 

 longitude of a place by merely observing its magnetic variation. The insuffi- 

 ciency of this method was, however, recognized at the time. Gilbert made a 

 similar proposal for determining longitude; but, instead of applying the varia- 

 tion, he thought to use the inclination or dip of the magnetic needle to obtain 

 the object sought. 



Enler, the great geometrician, also occupied himself with the theory of the 

 magnetism of the earth, and endeavored to show that the hypothesis of Halley 

 respecting four magnetic poles was unnecessary, and to prove from mathemati- 

 cal deduction that the assumption of the existence of two poles was sufficient; 

 he determined the position of them for the year 1757. The north pole was be- 

 yond latitude 76"" north, and longitude 96° west from Teneriffe; the south pole at 

 latitude 58° south, and longitude 158° west. 



In recent times a large number of the most accurate and valuable observa- 

 tions on the declination and inclination of the magnetic needle, and on the force 

 of terrestrial magnetism in different parts of the earth, and especially in the 

 neighborhood of the equator, have been made by Alexander von Humboldt 

 during his travels. It was principally from these observations that the French 

 physicist, Biot, endeavored to give an improved theory of the magnetism of the 

 eai-th. He assumes in this theory that the magnetic poles are not situated on 

 the earth's surface, but in its centre, and in close proximity to each other, and 

 by means of a somewhat complicated mathematical process he succeeds in 

 bringing the results of observations into apparent harmony with his theory. 



But one of the most zealous promoters of our knowledge of the magnetism 

 of the earth is Professor Christopher Hansteen, of Christiana, who, iu 1817, 

 published his work entitled " Investigations relative to the Magnetism of the 

 Earth." An incident in the beginning of the year 1807 gave the first impulse 

 to these investigations. Examining a physical globe constructed for the Cos- 

 mographical Society of Upsala, Hansteen found, at its south pole, an elliptic 

 figure, designated by the name of ''magnetic polar region,'' and it was further 

 inscribed on the globe that this naagnetic polar region had been delineated by 



