110 GENEEAL BEARINGS OF MAGNETIC OBSERVATIONS. 



included in the years 1835-1845 was "a time of unparalleled activity 

 in the extension of systematic and accurate magnetical observations 

 over the earth's surface." 



Whilst most of the continental nations joined in this movement, the 

 principal share in the work was divided between Germany, Russia, 

 and England in Europe, and the United States in America. But 

 before the splendid series of simultaneous observations made on the 

 continent, and four British colonial observatories were organized, 

 Gauss in 1839 published his general theory of terrestrial magnetism 

 coupled with a series of charts of the three magnetic elements for the 

 whole world, based upon observations made at ninety-two selected 

 stations distributed over the earth's surface; and it may be remarked 

 that Gauss's charts not only gave results in fair accordance with 

 observation in explored regions, but also as afterwards proved in 

 Antarctic latitudes hitherto un visited by man. 



The proof came in the years 1839-1843, when Boss's Antarctic 

 voyage of exploration was carried out in the interests of terrestrial 

 magnetism. Besides the importance of a knowledge of the general 

 distribution of the magnetic elements in those regions, one great aim 

 of this expedition was to reach the south magnetic pole. This was 

 found to be impossible, but sufficient data were collected to give its 

 approximate position. Whilst this Antarctic magnetic survey was 

 being completed, that of British North America was also undertaken, 

 with the result of the determination of the locality of one of the foci of 

 greatest intensity in the northern hemisphere. 



The results of these surveys formed, as will be well remembered, a 

 valuable series of "contributions" to terrestrial magnetism by Sabine, 

 and, coupled with every available observation between the years 1818 

 to 1876, formed the materials for the series of charts entitled " The 

 Magnetic Survey of the Globe" for the epoch 1842-1845. Each map 

 gave normal lines of equal values of the declination, inclination, and 

 intensity. Although it may be said that from the Arctic circle to the 

 Antarctic the direction of the lines was efficiently given by observa- 

 tion, the lines within those circles were largely taken from Gauss's 

 computed lines modified to agree with observation. 



Another difficulty in compiling these charts of Sabine's with accuracy 

 lay in the uncertain knowledge of the secular change then available, 

 and which had to be applied to observations made so far apart in time. 



Sabine's charts are doubtless the best we have for the epoch 1842-1845, 

 but in the light of the requirements of modern science they leave much 

 to be desired as regards the Antarctic; regions. The observations south 

 of 60° south were made entirely on board ships, where the errors of 

 the compass sometimes exceeded 50°, due to the horizontal forces in 

 the ship, thus rendering accurate observations of the declination very 

 uncertain and correction of the observed inclination very difficult; 

 besides which there are no records of the ship's disturbing force in the 

 vertical direction. 



