Magnetic Declination and Horizontal Force and Solar Spots. 69 



810. 



Length of Magnetic Period. 



13. 24. 35. 46. 57. 68. 79. 

 I2'55y ll-85y H'40y 10'45y ll'30y 13'(% 10'90y 



Length of Sun-spot Period. 

 12-50y 12-OOy ll'20y I0'50y ll'SOy 13'^Qy ll-2Qy 10'OOy 



The odd numbers, 1 3, 3 5, &c., indicate the periods between 

 successive minimum epochs, and the even numbers those between 

 successive maximum epochs. They are represented graphically in 

 fig. 2. The similarity of the variation in length of the complete 



FIG. 2. Length of Sun-spot and Magnetic Periods compared. 



K. 03 



10 



The thick line shows the variation in length of successive sun-spot periods, and the 

 thin line that between successive magnetic periods. Odd numbers indicate 

 periods from minimum to minimum, and even numbers periods from maximum 

 to maximum. 



magnetic and sun-spot periods, amounting to above three years, is 

 here well brought out. Mean magnetic period ll*42y. Mean sun- 

 spot period ll'57y. These values differ slightly from those above 

 given, owing to the numbers becoming here combined in a little 

 different way. Fig. 2 suggests a suspicion that the period decreases 

 in length through several periods, then increases for several periods, 

 and so on. But what may be the order of such variations is a 

 question to which at present it does not seem possible to give any 

 reliable answer. 



Examining further the collected curves, it is seen that the maximum 

 points of the curves have at different epochs very different degrees of 

 intensity. If for each curve we arrange the epochs of maximum in 

 order, with reference thereto, we find as follows : 



