36 THE OCEAN WORLD. 



If we draw a straight line from the moon passing through the centre- 

 of the earth, this line will meet the surface of the waters at two points 

 diametrically opposite to each other namely, z and N (Fig. 5) ; one 

 of these points would be to the moon its zenitli, the other its nadir. 

 The point of the sea which has the moon in the zenith namely, 

 that above which the moon is perfectly perpendicular will be nearest 

 to the planet, and will consequently be more strongly attractive to the 

 centre of the earth, while the points diametrically opposite to which 

 the moon is the nadir will be more distant, and consequently less 

 strongly attracted by that luminary. It follows that the waters 

 situated directly under. the moon will be attracted towards it, and 

 form an accumulation or swelling at that point ; the waters at the 

 antipodes being less strongly attracted to the moon than to the centre 

 of the earth, will form also a secondary swelling on the surface of the 

 sea, thus forming a double tide, accumulating at the point nearest the 

 moon and at its antipodes. At the intermediate points of the cir- 

 cumference of the globe, where the waters are not subjected to the 

 direct attraction of the moon, the sea is at low water, as represented 

 in Fig. 5. 



The earth, in its movement of rotation, presents, in the course of 

 twenty-four hours, every meridian on its surface to the lunar attrac- 

 tion ; consequently, each point in its turn, and at intervals of six hours, 

 is either under the moon, or ninety degrees removed from it : it 

 follows, that in the space of a lunar day that is to say, in the 

 time which passes between two successive passages of the moon on 

 the same meridian the oceanic waters will be at high and low tide 

 twice in the month on every point of the surface of the. globe. But 

 this result of attraction is not exercised instantaneously. The moon 

 has passed from the meridian of the spot before the waters have attained 

 their greatest height ; the flux reaches its maximum about three hours 

 after the moon has culminated ; and the watery mountain follows the 

 moon all round the globe, from east to west, about three hours in its 

 rear. 



It is obvious, however, that the great inequalities of the bottom of 

 the sea ; the existence of continents ; the slopes of the coast, more or 

 less steep ; the different breadths of channels and straits ; finally, the 

 winds, the pelagic currents, and a crowd of local circumstances, must 

 materially modify the course of the tides. Nor is the moon the only 



