60 COSMOS. 



received with much favour by physicists, have been formed 

 on a similar plan to Halley's isogonic curves. These lines, 

 especially since they have been extended and greatly im- 

 proved by Dove, are intended to afford a clear view of the 

 distribution of heat on the earth's surface, and of the princi- 

 pal dependence of this distribution on the form of the solid 

 and fluid parts of the earth, and the reciprocal position of 

 continental and oceanic masses. Halley's purely scientific 

 expeditions stand so much the more apart from others, since 

 they were not, like many later expeditions, fitted out at the 

 expense of the Government with the object of making geo- 

 graphical discoveries. In addition to the results which they 

 have yielded 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. Helena in the years 1677 and 1678. This 

 catalogue was moreover the first that was drawn up after 

 telescopes had been combined, according to Morin's and 

 Gascoigne's methods, with instruments of measurement. 65 



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 characterised by 

 the discovery of horary periodical alterations of variation. 

 Graham has the incontestable merit of being the first to 

 observe (London, 1722) these hourly variations with accuracy 

 and persistency. Celsius and Hiorter in Upsala, 66 who main- 

 tained a correspondence with him, contributed to the exten- 

 sion of our knowledge of this phenomenon. Brugmans, and 

 after him Coulomb, who was endowed with higher mathe- 

 matical powers, entered profoundly into the nature of ter- 



65 Twenty years after Halley had drawn up his catalogue of southern 

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

 sixth magnitude) Hevelius boasted, in his Firmamentum Sobescianum, 

 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. 52. 



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

 had been observed in London as early as 1634, by Hellibrand, and in 

 Slam, by Father Tachard, in 1682. 



