120 ELEMENTARY LESSONS ON [CHAP. n. 



142. Daily Variations. Both compass and dipping- 

 needle, if minutely observed, exhibit slight daily motions. 

 About 7 a.m. the compass needle begins to travel west- 

 ward with a motion which lasts till about i p.m. ; during 

 the afternoon and evening the needle slowly travels back 

 eastward, until about 10 p.m. ; after this it rests quiet ; 

 but in summer-time the needle begins to move again 

 slightly to the west at about midnight, and returns again 

 eastward before 7 a.m. These delicate variations never 

 more than i o ; of arc appear to be connected with the 

 position of the sun ; and the moon also exercises a 

 minute influence upon the position of the needle. 



143. Annual Variations. There is also an annual 

 variation corresponding with the movement of the earth 

 around the sun. In the British Islands the total force 

 is greatest in June and least in February, but in the 

 Southern Hemisphere, in Tasmania, the reverse is the 

 case. The dip also differs with the season of the year, 

 the angle of dip being (in England) less during the four 

 summer months than in the rest of the year. 



144. Eleven -Year Period. General Sabine dis- 

 covered that there is a larger amount of variation of the 

 declination occurring about once every eleven years. 

 Schwabe noticed that the recurrence of these periods 

 coincided with the eleven -year periods at which there 

 is a maximum of spots on the sun. Professor Balfour 

 Stewart and others have endeavoured to trace a similar 

 periodicity in the recurrence of aurorae 1 and of other 

 phenomena. 



145. Magnetic Storms. It is sometimes observed 

 that a sudden (though very minute) irregular disturbance 

 will affect the whole of the compass needles over a con- 

 siderable region of the globe. Such occurrences are 

 known as magnetic storms ; they frequently occur at 

 the time when an aurora is visible. 



146. Self-recording Magnetic Apparatus. At 



1 See Lesson XXIV., on Atmospheric Electricity. 



