THE TIDES. 



it varies greatly at different places, and its laws, which appear 

 to be governed by local circumstances, are very imperfectly 

 known. 



11. We have now described the principal phenomena that would 

 take place were the earth a sphere, and covered entirely with a 

 fluid of uniform depth. But the actual phenomena of the tides 

 are infinitely more complicated. From the interruption of the 

 land, and the irregular form and depth of the ocean, combined 

 with many other disturbing circumstances, among which are the 

 inertia of the waters, the friction on the bottom and sides, the 

 narrowness and length of the channels, the action of the wind, 

 currents, difference of atmospheric pressure, &c. &c., great 

 variation takes place in the mean times and heights of high 

 water at places differently situated ; and the inequalities above 

 alluded to, as depending on the parallax of the moon, her position 

 with respect to the sun, and the declination of the two bodies, 

 are, in many cases, altogether obliterated by the effects of the 

 disturbing influences, or can only be detected by the calculation 

 and comparison of long series of observations. 



12. According to Dr. Whewell, the general progress of the great 

 tide-wave may be thus described : It is only in the Southern 

 ocean, between the latitudes of 30 and 70, that a zone of water 

 exists of sufficient extent to allow of the tide-wave being formed. 

 Suppose, then, a line of contemporary tides, or cotidal line, to be 

 formed in the Indian ocean, as the theory supposes, that is to 

 say, in the direction of the meridian, and at a certain distance 

 to the eastward of the meridian in which the moon is. As this 

 tide- wave passes the Cape of Good Hope, it sends off a derivative 

 undulation, which advances northward up the Atlantic ocean, 

 preserving always a certain proportion of its original magnitude 

 and velocity. In travelling along this ocean the wave assumes 

 a curved form, the convex part keeping near the middle of the 

 ocean, and ahead of the branches, which, owing to the shallower 

 waters, lag behind on the American and African coasts, so that 

 the cotidal lines have always a tendency to make very oblique 

 angles with the shore, and, in fact, run parallel to it for great 

 distances. The main tide, Dr. Whewell conceives, after reaching 

 the Orkneys, will move forward in the sea bounded by the shores 

 of Norway and Siberia on one side, and those of Greenland 

 and America on the other, will pass the pole of the earth, 

 and finally end its course on the shores in the neighbourhood- of 

 Behring's Straits. It may even propagate its influence through 

 the ^straits, and modify the tides of the North Pacific. But a 

 -branch tide is sent off from this main tide into the German 

 ocean ; and this, entering between the Orkneys and the coast of 



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