THE SUN'S WORK. 351 



which, consequently, can no longer be attributed to that force. We 

 see them drawn out and lengthened in the direction of the above- 

 mentioned ideal line ; but the matter which gushed forth (so to speak) 

 on the sunshiny side is quickly arrested and driven back, while on the 

 opposite side the matter rushes out, without meeting an obstacle, with 

 an impetuosity of which no earthly phenomenon can give an idea. In 

 a few days there is thus produced a tail diametrically opposite to the 

 sun ; and this tail may stretch before our eyes to a length of ten, 

 twenty, thirty, or even sixty millions of leagues. 



So great, in truth, is the complexity of cosmical phenomena, even 

 when connected with unknown forces, that we might despair of suc- 

 cess were we not offered a resource of which astronomical science has 

 often availed itself namely, the comparison of phenomena which occur 

 at the same periodical intervals. Long before the discovery of uni- 

 versal gravitation, it was easy to see that the tides depended on the 

 moon, since the periodicity is identical for the oscillations of the sea 

 and the movements of our satellite. In like manner, the most com- 

 plicated phenomena of meteorology, if they manifest a periodical char- 

 acter and their course agrees with the period of other strange phe- 

 nomena, betray thereby their connection, in some way or other, with 

 the latter. 



Let us take, as an illustration, the variations of the dip of the mag- 

 netic needle. Every day, that needle, suspended freely, deviates in 

 the morning from the position of equilibrium, and every day returns 

 to it in the evening, after an excursion of variable extent. These 

 regular movements evidently depend on tl)e presence of the Sun above 

 the horizon of any given spot ; they also depend on its geographical 

 situation, for they increase with the latitude on one hemisphere, and 

 change their direction in passing from one hemisphere to the other. 

 They are not due to a magnetic action proper to the sun ; for, even 

 supposing it to exist, the Sun, in consequence of his enormous distance, 

 would be incapable of exercising a directing influence on a magnetic 

 needle ; but they result from some unknown action exerted on the 

 electricity of the globe, and on the currents resulting from it cur- 

 rents which themselves react on the direction of the needle, and often 

 seriously disturb its movements. 



How are we to give a precise account of this mode of action ? In 

 the midst of so many unknown details, how are we to lay hold of those 

 which really require our attention ? Observation, only, aided by this 

 special form of empiricism pointed out by M. Faye in his " Notice," 

 and here attempted to be described can help us in the matter. The 

 diurnal variations of the needle have been noted for nearly a century 

 past ; it is remarked that they are not constant from one year to 

 another ; that they present maxima and minima, epochs of greatest 

 and least activity ; that these maxima occur every eleventh year. 

 The phenomenon is pei'iodical, and its period is eleven years. 



