THE 



VOYAGE OF H.M.S. CHALLENGER. 



PHYSICS AND CHEMISTRY. 



REPORT on the Magnetical Results obtained by H.M.S. Challenger 

 during the years 1873-1876. By Staff-Commander E. W. Creak, 

 R.N., F.R.S. 



Since the year 1700, when Halley published his map of equal curves of magnetic 

 declination for the Atlantic and Indian Oceans, that method of representing the values 

 of the magnetic elements for frequent reference seems to have found general favour, 

 probably from the facility with which the information they contain can be utilised. 

 Thus since Halley's day the following authors have published maps of the declination : — 

 Mountaine andDodson in 1756, Churchman in 1794, Yeatesin 1817, and Barlow in 1833. 



Again, in 1819, Hansteen 1 added maps of the inclination to those of the declination 

 for different epochs between the years 1600 and 1787, and although the angular direction 

 of the freely suspended needle was thus known for a considerable portion of the earth's 

 surface, it was not until the present century that the intensity of the earth's directive 

 force was observed and known as well as the other elements. In the year 1826 he 

 published a chart of " Isodynamic Lines," which he revised in 1832, both editions beino- 

 based partly upon his own observations, combined with those from other available 

 sources. 



Following Hansteen there appeared in 1840 Gauss and Weber's Atlas, the result of 

 calculation from about eighty-four observations distributed over the world. Considering 

 the comparatively slender basis of observation upon which they had to rely for the 

 application of their mathematical investigations, it is remarkable, even when regarding 

 their work in the light of the results of the present activity amongst magnetic observers, 

 how nearly they approached the truth. It may be added that, in view of the extended 

 knowledge now possessed of the distribution of the earth's magnetism, there remains 



1 Magnetismus der Erde. 



(PHYS. CHEM. CHALL. EXP. PART VI. 1888.) 1 



