2 INTRODUCTION. 



west of Greenwich.^ From Philadelphia, Toronto bears 38° 45' west of north (true), 

 and is distant 4° 50' in arc, or about 334 statute miles. 



It is proposed in the present paper to investigate the law of the eleven year 

 period, or as it is more frequently called, the decennial period, there being yet an 

 uncertainty as to its precise length. It is supposed to have some direct or indirect 

 connection with the solar spot period, which, according to late investigations by 

 Prof R. Wolf," is said to exhibit corresponding disturbances. 



The discussion is a contribution towards the determination of the epoch of the 

 occurrence of a minimum (as to number and magnitude) in certain phases of the 

 magnetic variations and disturbances, corresponding to a minimum in the solar 

 spot period. The method of reduction is substantially the same as that adopted 

 by General Sabine, and explained in his discussion of the Toronto and Hobarton^ 

 observations. 



' This longitude depends on that of Cambridge observatory, for which 4'' 44™ 30'. 25 has been adopted. 

 ° Astronomische Nachrichten, No. 1091 (May, 1857). 



' See three papers by General Sabine, on periodical laws discoverable in the mean effects of the larger 

 magnetic disturbances. Philosophical Transactions of the Royal Society, 1851, 1852, and 1856. 



