TERRESTRIAL MAGNETISM. 297 



(<r) The secular variation. Each magnetic element is subject 

 to a slow change from year to year. This is called the secular 

 variation. Thus, the curve in Fig. 7 shows the secular variation 

 of the magnetic declination at London from 1540 to 1890. 



(d} Magnetic storms. Each magnetic element is subject to 

 erratic variations. These erratic variations occur at times of great 

 disturbances in the sun as indicated by sun-spot activity, and also 

 at times of great disturbances in the earth such as volcanic erup- 

 tions and earthquakes, and they are called magnetic storms. 



