66 THE PRINCIPLES OF SCIENCE. 



cause. Accordingly whenever we find any two phenomena 

 which do proceed, time after time, through changes of 

 exactly the same period, there is much probability that 

 they are connected. It was in this manner, doubtless, that 

 Pliny correctly conjectured that the cause of the tides 

 lay in the sun and moon, the intervals between suc- 

 cessive high tides being equal to the intervals between 

 the moon's passage across the meridian. Kepler and 

 Descartes too admitted the connection previous to 

 Newton's demonstration of its precise nature. When 

 Bradley discovered the apparent motion of the stars 

 arising from the aberration of light, he was soon able to 

 attribute it to the earth's annual motion, because it went 

 through all its phases in exactly a year. 



The most extensive and beautiful instance of induction 

 concerning periodic changes which can be cited, is that of 

 the discovery of an eleven-year period in various meteoro- 

 logical and astronomical phenomena. It would be difficult 

 to mention any two things apparently more disconnected 

 than the spots upon the sun and auroras. As long ago as 

 1826, Schwabe, of Dessau, commenced a regular series of 

 observations of the spots upon the sun, which has been 

 continued to the present time, and he was able to show 

 that at intervals of about eleven years the spots increased 

 much in size and number. Hardly was this discovery 

 made known, than Dr. Lamont pointed out a nearly equal 

 period of variation in the magnetic needle as regards 

 declination. The occasional magnetic storms or sudden 

 irregular disturbances of the needle were next shown to 

 take place most frequently at the times when sun spots 

 were prevalent, and as auroras are generally coincident 

 with magnetic storms, these strange phenomena were 

 brought into the cycle x . It has since been shown by 



* 'Nature,' vol. i. p. 284; Quetelet, 'Sur la Physique du Globe,' 

 pp. 148, 262-64, &c. 



