MAGNETISM AT HALIFAX, AUGUST, 1904. — DIXON. 247 



maker and hydrographer of London. Norman published his di.s- 

 covery in a treatise called " The Newe Attraction," printed in 

 1581, and gives the value of the dip at London in 1576 as 

 71" 50'. Norman's discovery had been anticipated by George 

 Hartmann, for in a letter to Count Albert of Prussia, March 4, 

 1544, in which he recounts his observations of the declination, 

 he mentions tiiat the needle, besides deflecting towards the east, 

 also pointed downwards. Hartmann, however, did not suspend 

 the needle so as to observe the dip correctly and only recorded 

 an amount of 9° instead of about 65", the value he ought to 

 have found. 



Intensity. 



The third element, the Intensity of the earth's magnetic 

 force, was first measured by William Whiston. In 1720 a prize 

 was offered for the best way of determining longitude at sea. 

 Columbus, in 1592, had first suggested using the lines of equal 

 declination for determining longitudes, but Gilbert, who pub- 

 lished his great work " De Magnete " in 1600, proposed to 

 determine longitude by variations in the angle of dip. Whiston, 

 taken with this latter theory, constructed many dip circles, and 

 as a result of his observations, showed how the intensity with 

 which the earth attracted a magnetic needle varied from place 

 to place. As a matter of convenience, it is the horizontal com- 

 ponent of the earth's magnetic force that is measured, and the 

 unit in which the measurement is made is called a gauss, after 

 the celebrated physicist who first showed how the absolute 

 intensitj^ of the earth's magnetism might be found. 



Secular Change in Declination. 



A magnetic survey of a country aims at making determina- 

 tions of the three elements at a sufiicient number of suitable 

 localities. But when such a survey has been completed, if the 

 value of any one of the elements is compared with that deter- 

 mined at the same place on some previous occasion, a difference 



