GENERAL BEARINGS OF MAGNETIC OBSERVATIONS. 109 



these errors to insure the safe navigation of the ships, but it had also a 

 direct bearing - in enabling observers to eliminate the hitherto inexpli- 

 cable divergencies in the values of the declination observed in different 

 ships in the same geographical position. The results of these experi- 

 ments bore no immediate fruit, for with the death of Flinders the sub- 

 ject was temporarily neglected. 



In 1819, Hansteen published his Magnetismus der Erde 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 conclusions 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 geographical 

 positions. Further than this, he computed that to account for the 

 secular change these four supposed poles revolved round the terres- 

 trial 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 distribu- 

 tion of that magnetic element was so little known that we may witk 

 our x>resent 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 favor 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 j)oles 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 toward the solution of the 

 problem of the ever variable 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 three magnetic elements became 

 a necessity in solving the problems which the magnetism of different 

 iron ships presented. 



With Gauss's invention of the absolute horizontal force magneto- 

 meter in 1833, many hitherto unknown movements of the magnetic 

 needle of the highest interest were discovered, which with the coarser 

 instruments previously in use lay concealed. This discovery gave the 

 desired impetus to the scientific men of that epoch, and the period 



