THE MAGNETIC POLES. 61 



in respect to terrestrial magnetism, they were also the means 

 of affording us an important catalogue of southern stars as 

 the fruits of Halley's earlier sojourn in the island of St. He- 

 lena in the years 1677 and 1678. This catalogue was, more- 

 over, the first that was drawn up after telescopes had been 

 combined, according to Morin's and Gascoigne's methods, 

 with instruments of measurement.* 



As the 17th century had been distinguished by an advance 

 in a more thorough knowledge of the position of the lines of 

 variation, and by the first theoretical attempt to determine 

 their points of convergence, viz., the magnetic poles, the 18th 

 century was characterized by the discovery of horary period- 

 ical alterations of variation. Graham has the incontestable 

 merit of being the first to observe (London, 1722) these hour- 

 ly variations with accuracy and persistency. Celsius and Hi- 

 orter in Upsala,f who maintained a correspondence with him, 

 contributed to the extension of our knowledge of this phe- 

 nomenon. Brugmans, and after him Coulomb, who was en- 

 dowed with higher mathematical powers, entered profoundly 

 into the nature of terrestrial magnetism (1784-1788). Their 

 ingenious physical experiments embraced the magnetic attrac- 

 tion of all matter, the local distribution of the force in a mag- 

 netic rod of a given form, and the law of its action at a dis- 

 tance. In order to obtain accurate results, the vibrations of 

 a horizontal needle suspended by a thread, as well as deflec- 

 tions by a torsion balance, were in turn employed. 



The knowledge of the difference of intensity of terrestrial 

 magnetism at different points of the earth's surface by the 

 measurement of the vibrations of a vertical needle in the 

 magnetic meridian is due solely to the ingenuity of the Cheva- 

 lier Borda — not from any series of specially successful ex- 

 periments, but by a process of reasoning, and by the decided 

 influence which he exerted on those who were equipping 

 themselves for remote expeditions. Borda's long-cherished 

 conjectures were first confirmed by means of observations 



* Twenty years after Halley had draAvn up his catalogue of south- 

 ern stars at St. Helena (which, unfortunately, included none under 

 the sixth magnitude) Hevelius boasted, in his Firmamentum Sobescia- 

 num, that he did not employ any telescope, but observed the heavens 

 through fissures. Halley, who, during his visit to Dantzic in 1679, 

 was present at these observations, praises their exactness somewhat 

 too highly. Cosmos, vol. iii., p. 42. 



t Traces of the diurnal and horary variations of the magnetic force 

 had been observed in London as earlv as 1634. by Hellibrand, and in 

 Siam by Father Tachard, in 1682. 



