ON THE PROGRESS OP SCIENCE. XIX 



months are the epochs of maximum disturbance, and the solstitial 

 months epochs of minimum disturbance. It has also been discovered 

 that the occurrence of the larger disturbances of the vertical force at 

 Toronto is governed by periodical laws depending on the hours of so- 

 lar time. The a^re^ate value of the disturbances in the five years 



C^O O 



is a maximum at 3 p. M. and a minimum at 1 1 A. M. There is also a 

 secondary maximum at 5 p. M. and a secondary minimum at 9 p. M. 



The three magnetic elements concur in showing that the moon exer- 

 cises a sensible magnetic influence at the surface of the earth, produc- 

 ing in every lunar day a variation in each of the three elements ; but 

 by far the most interesting discovery connected with terrestrial mag- 

 netism is the curious accordance between intense magnetic disturbance 

 and spots on the sun. These spots have been observed to increase 

 and decrease in number and intensity decennially, and it appears that 

 the periodical magnetical inequality has its opposite phases of maxi- 

 mum and minimum separated by an interval of five years, of which 

 the cycle might therefore be conceived to include about ten of our so- 

 lar years. Respecting this remarkable circumstance, General Sabine 

 observes : 



" Had no other circumstance presented itself to give additional in- 

 terest to an investigation which held out at least a fair promise of 

 making known laws of definite order and sequence in phenomena 

 which have excited so much attention of late years, but of which so 

 little has hitherto been ascertained, had, for example, the decennial 

 period which appeared to prevail with precisely corresponding feat- 

 ures in two distinct classes of the magnetic variations, connected itself 

 with no other periodical variation either of a terrestrial or cosmical 

 nature with which we are acquainted, there might have been, indeed, 

 little reason to apprehend, in these days of physical curiosity and in- 

 ductive application, that the investigation would have been suffered to 

 drop ; but the interest has doubtless been greatly enhanced by the re- 

 markable coincidence, between the above-described periodical inequal- 

 ity by which the magnetic variations referable to solar influence are 

 affected, and the periodical inequality which has been discovered by 

 3,1. Schwabe to exist in the frequency and magnitude of the solar spots. 

 The coincidence as far as we are yet able to discover, is absolute ; the 

 duration of the period is the same, and the epochs of maximum and 

 minimum fall in both cases on the same years. The regularity with 

 which the alternations of increase and decrease have been traced by 

 M. Schwabe in his observations of the solar spots (which have been 

 now continued for about thirty years), must be regarded as conferring 

 a very high degree of probability on the systematic character of causes 

 which as yet are known to us only by the visible appearances which 

 they produce on the sun's disk, and by the disturbances which they oc- 



