40 
THE OCEAN WORLD. 
diametrically opposite to each other — namely, z and n (Fig. 5) ; one 
of these points would he to the moon its zenith, 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 he 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 
celestial body which influences the rise and fall of the waters of the sea. 
We have already said that the sun asserts an influence on the waves. 
