84 SCIENCE PROGRESS. 



with an atlas containing charts of the elements Declination 

 and Dip for different epochs between the years 1600 and 

 1787. These charts were in a large measure compiled from 

 observations made with imperfect instruments and subject 

 to the causes of error already mentioned attending both land 

 and sea results. Hansteen, however, considered them of 

 sufficient value to enable him to draw certain important con- 

 clusions with regard to the cause of the secular change of 

 the magnetic elements. Thus he not only concurred with 

 Halley that the earth considered as a magnet had four 

 poles or points of attraction, but computed their geo- 

 graphical positions. Further than this, he computed that 

 to account for the secular change these four supposed 

 poles revolved round the terrestrial poles, each pole 

 occupying a widely different number of years to complete 

 the revolution. 



If these theoretical results had been true, a great 

 advance would have been made not only in the science 

 of terrestrial magnetism but in its practical bearing on the 

 requirements of the present day. 



Although Humboldt had about the year 1800 shown 

 that the intensity of the earth's magnetism varied with the 

 latitude, the general distribution of that magnetic element 

 was so little known that we may with our present extended 

 knowledge consider that Hansteen's conclusions were based 

 on insufficient data. In fact the idea of the earth being a 

 magnet with four poles has long since been abandoned in 

 favour of there being one pole with two foci of intensity in 

 each hemisphere, and reasons will be given further on 

 which tend to throw doubt on there being any revolution of 

 these two magnetic poles round their adjacent terrestrial 

 poles. 



Subsequently to Hansteen's charts there appeared those 

 of the Declination by Yeates, Duperrey, and by Barlow in 

 1836. These were useful to navigation but helped very 

 little towards the solution of the problem of the ever vari- 

 able distribution of the earth's magnetism. 



Besides this by the year 1835 the iron-built ship had 

 appeared on the ocean and a correct knowledge of the 



