APPENDIX A. 

 TERRESTRIAL MAGNETISM.* 



1. The earth a great magnet. The tendency of a compass 

 needle to set itself in a particular direction at a given place on the 

 earth was at a. very early date attributed to some action of the 

 earth. The famous Dr. Gilbert, Physician in Ordinary to Queen 

 Elizabeth, in his Latin treatise f put forward the important idea 

 that the earth is a great magnet, so that, in the language of 



/axis of 

 'rotation 



Ftg. 1. 



Faraday, there exists a magnetic field around the earth. The 

 general character of the earth's magnetic field as to its direction 



* A fairly complete discussion of terrestrial magnetism with many important refer- 

 ences is given in Gray's Treatise on Magnetism and Electricity, Vol. I, pages 58-84, 

 Macmillan and Company, 1898. 



f De Magnete magnetidsque corporibus. Translated into English about 1902 . 



292 



