Departure. — Theory of lliirrleanes. 1 83 



a mere question of the study of local conditions in order to be 

 able to declare how these universal laws operate, and to eluci- 

 date by the most simple explanations many of the phenomena of 

 nature that have till now baffled science. Thus, when a wind 

 hitherto steady shifts its direction, there must necessarily be 

 certain active causes for its doing so ; if these causes perpetually 

 recur in well-marked periodical intervals, the change of the wind 

 must follow a definite law. Under certain circumstances the 

 direction of the wind is well-defined ; as, for instance, at 

 certain seasons in the open ocean it remains always the same, 

 or changes with a certain regularity, whence it becomes 

 apparent that the causes must remain unchanging, and the 

 recurrence of the phenomenon must accordingly admit of 

 explanation. 



We know, for example, that in the case of hurricanes — 

 those most terrific exemplifications of the tendency of the 

 atmosphere to move in circles — the wind does not blow in 

 straight lines, but rather in curves described round a central 

 point, which again is not immovable, but has a regular pro- 

 gression along a definite curve. In that curved plane, how- 

 ever, which has been termed a cyclone, the wind always blows 

 in one and the same direction, and in the Northern Hemi- 

 sphere runs counter to the motion of a watch-hand, while in 

 the Southern Hemisphere it, on the contrary, follows that 

 motion. 



These facts once granted as accounting for such phenomena, 

 it follows as a natural consequence of the general principles 



