106 LIGHT SCIENCE FOR LEISURE HOURS. 



not to the mere strength of the moon's * pull ' upon 

 the waters of any ocean that the tidal wave owes its 

 origin, but to the difference of the forces by which the 

 various parts of that ocean are attracted. The whole 

 of an ocean cannot be raised at once by the moon; but 

 if one part is attracted more than another, a wave is 

 formed. That this may happen, the ocean must be one 

 of wide extent. In the vast seas which surround the 

 Southern Pole there is room for an immensely powerful 

 * drag,' so to speak ; for always there will be one part of 

 these seas much nearer to the moon than the rest, and 

 so there will be an appreciable difference of pull upon 

 that part. 



The reader will now see why I have been so- 

 careful to ascertain the limits of the supposed north- 

 polar ocean, in which, according to Captain Maury> 

 tidal waves are generated. To accord with his views, 

 this ocean must be surrounded on all sides by impass- 

 able barriers either of land or ice. These barriers,, 

 then, must lie to the northward of the regions yet 

 explored, for there is open sea communicating with 

 the Pacific all round the north of Asia and America. 

 It only requires a moment's inspection of a terrestrial 

 globe to see how small a space is thus left for Captain 

 Maury's land-locked ocean. I have purposely left 

 out of consideration, as yet, the advances made by 

 arctic voyagers in the direction of the sea which 

 lies between Greenland and Spitzbergen. We shall 

 presently see that on this side the imaginary land- 

 locked ocean must be more limited than towards the 



