PHYSICAL ASTRONOMY. 129 
the falling the ebb. In this manner the whole phenomenon is incessantly 
repeated in periods of 12 hours and 25 minutes. The difference of elevation 
of the water at high tide and at low tide is not the same in all places at the 
same time, nor in one and the same place at all times. This difference, fou 
example, on the German coast of the North Sea, amounts to 13 feet ; while 
at the western end of the Straits of Dover it is sometimes more than 46 
feet. The position of the coast, and the direction of the wind, change not 
a little the regular course of the whole phenomenon. Apart, however, from 
these local and temporary influences, a monthly and a yearly period are 
plainly evident. There is a greater difference between high and low water 
at the time of new and full moon, than at the time of the quadratures; and 
furthermore, this difference is more considerable when the sun and earth are 
nearest to each other, than when most remote. From this there cannot be 
the slightest doubt that the sun, and more particularly the moon, produce 
the ebb and flow of the tide. 
For the proper elucidation of this phenomenon, suppose the earth’s sur 
face to be covered equally with water, and let us inquire what shape this 
watery surface will assume when the earth, on account of the attraction. 
begins to fall towards the moon; we will here have reference only to the 
influence of the latter, as being the most important. It is evident that those 
portions of the water will be attracted the strongest, which lie nearest the 
moon M (pl. 6, fig. 23), and consequently the water surrounding the earth 
will be heaped up highest at that place, O, which has the moon in its zenith. 
Here, then, where a mountain of water has arisen, the height of the water 
will be greatest, decreasing, however, more and more in every direction, 
reaching the minimum at those points, Z and Z’ (at time of full moon V and 
V’), which have the moon in the horizon ; at the point O’ of the earth, which 
has the moon in its nadir, there will also be an elevation of the water: this 
point will be attracted least, and consequently will remain further behind 
the other points. Hence it foilows that there will be a rise of the water at 
the two points, O, O’, of the earth, distant a whole diameter from each other, 
and lying in the straight line connecting the centres of the earth and moon. 
From these two points, the height of the water will decrease according to a 
certain law, until finally it will be lowest in the points of that great circle 
which has the two points of highest water for its poles; that is, as before 
mentioned, in all those places which have the moon in their horizon. Now, 
although the earth is not a globe entirely surrounded by water, yet by far 
the greatest portion is covered with water; and the waters of the sea will 
thus be heaped up in those points which have the moon in the meridian, 
whether at the inferior or superior culmination. Since the moon, on ac- 
count of its own motion and the rotation of the earth, culminates every 12 
hours and 25 minutes for one and the same place, the phenomenon known 
as the ebb and flow must continually return within this period. Neverthe- 
less, the time of flood does not exactly coincide with that of the culmination 
of the moon, which at first may appear strange ; but when we reflect that on 
account of the inertia of the material, the mass of water cannot immediately 
follow the apparent motion of the moon, that apparent anomaly will be 
ICONOGRAPHIC ENCYCLOPDIA.—VOL. I, 9 129 
