DECLINATION. 257 



different times. If we travel to the west the variation increases westerly, 

 and is greatest in the Atlantic. It then decreases ; the needle points due 

 north in North America. Going still forward the variation becomes easterly, 

 increases, and decreases to nothing in Eastern Russia. Thence the variations 

 are westerly. Columbus discovered the variation of the compass needle in 

 September 1493. In places where the needle is due north and south the 

 lines drawn through them are termed lines of no variation. 



The variation, however, is not always the same in the same place. In 

 the year I 580, in London, for instance, the variation was 1 1 1 1' E. A little 

 more than one hundred years later London was on the line of no variation, 

 and now the tendency is westerly. On the other hand, there are places where 

 there is no deviation, and Sir John Herschell says that West India property 

 has been saved from litigation in consequence of the invariability of magnetic 

 declination there, for all surveys were made by the compass. Lines of equal 

 variation are called isogonial; those of equal dip or inclination, isoclinical; 

 and those of equal intensity, isodynamical. 



As we have said, the magnetic elements are not always the same, and 

 the variations of the compass are daily and annually observed with certain 

 instruments. What are termed secular variations take place at long 

 intervals, and the following table will show : 



In 1576 the angle of declination in England was 11 15 East. 



reached in 

 , and was 



22 34'. The rate of decrease is about 8' a year, but varies in different 

 periods, as may be seen. The discovery of the fact that an annual 

 variation took place in the angle of declination, is attributable to Cassini, 

 and the diurnal variation was discovered by Graham in 1722. From 

 8 o'clock in the morning, when the needle is pointing a little to the 

 east of its "mean position," it turns towards the west until I p.m. It then 

 returns towards the east again, and passing westerly again between midnight 

 and three o'clock a.m., settles down till eight a.m., when it begins afresh. 

 This variation does not apply to all places. 



Magnetic inclination is besides subject to changes. There are also- 

 variations of magnetic force which occur at very irregular periods, and 

 cannot be said to follow any laws. These disturbances are called Magnetic 

 Storms, of which the Aurora Borealis is one result. 



17 



