MAGNETIC APPARATUS. 149 



investigated experimentally by Coulomb, and the mathematical 

 expression deduced by Biot from his results has been verified for 

 the same case by various subsequent investigators, who have for 

 the most part employed methods depending upon the laws of 

 magneto-electric induction. Chiefly by similar methods, the dis- 

 tribution of magnetism in some other cases has been examined, 

 and investigations have been made into the relation between the 

 degree of magnetisation and the intensity of the magnetising 

 force, as well as into the connection between magnetisation and 

 various other conditions, such, for example, as different kinds of 

 mechanical strain. 



An extremely important branch of magnetic science is that 

 which deals with the magnetic properties of the earth. There is 

 no evidence as to the first discovery of the directive properties of 

 the magnet, but the use of a magnetic needle in navigation was 

 certainly known in Western Europe at least as early as the 

 beginning of the thirteenth century. The fact that a compass- 

 needle does not point in the same direction at all parts of the 

 earth's surface was discovered by Columbus during his first 

 voyage, on the ijth of September, 1492.* It has already been 

 mentioned in this article that the same fact was known fifty years 

 later to G. Hartmann, who had observed the compass to point 6 

 in Rome and 10 in Nuremberg to the west of North. The pro- 

 perty of a compass-needle supported at its centre of gravity to 

 4t dip," as though one end had become heavier by magnetisation, 

 was also, as has been said, observed by Hartmann ; but the first 

 tolerably accurate measurement of the magnetic dip is due to 

 Robert Norman, an English instrument-maker, who in 1576, 

 found the dip in London to be 71 50'. 



Up to Gilbert's time, such observations as these seem to have 

 been regarded simply as throwing light upon the intrinsic pro- 

 perties of the loadstone, or of artificial magnets, but when it was 

 recognised that the phenomena in question were in. reality indica- 



* Arago's Meteorological Essays, translated by Sabine, p. 323. 



