THE TIDES. 



GRAVITATION was established by Newton, and its laws fully 

 developed, that all the circumstances of the tides were clearly 

 explained, and shown incontestably to depend on the influence of 

 the sun and moon. 



2. There are few siibjects in physical science about which there 

 prevail more erroneous notions among those who are but a little 

 informed, than with respect to the tides. A common idea is, 

 that the attraction of the moon draws the waters of the earth 

 toward that side of the globe on which it happens to be placed, 

 and that consequently they are heaped up on that side, so that 

 the oceans and seas acquire there a greater depth than else- 

 where ; and thus it is attempted to be established that high 

 water will take place under, or nearly under, the moon. But 

 this neither corresponds with the fact, nor, if it did, would it 

 explain it. High water is not produced merely under the moon, 

 but is equally produced upon those parts most removed from the 

 moon. Suppose a meridian of the earth so selected, that if its 

 plane were continued beyond the earth, it would pass through 

 the moon ; then we find that, subject to certain modifications, a 

 great tidal wave, or what is called high water, will be formed on 

 both sides of this meridian ; that is to say, on the side next the 

 moon, and on the side remote from the moon. As the moon 

 moves in her monthly course round the earth, these two great 

 tidal waves follow her. They are, of course, separated from each 

 other by half the circumference of the globe. As the globe 

 revolves with its diurnal motion upon its axis, every part of its 

 surface passes successively under these tidal waves ; and at all such 

 parts as pass tinder them, there is the phenomenon of high water. 

 Hence it is that in all places there are two tides daily, having an 

 interval of about twelve hours between them. Now if the common 

 notion of the cause of the tides were well founded, there would be 

 only one tide daily; viz., that which would take place when the 

 moon is at or near the meridian. 



3. That the moon's attraction upon the earth simply considered 

 would not explain the tides, is easily shown. Let us suppose 

 that the whole mass of matter on the earth, including the waters 

 which partially cover it, were attracted equally by the moon ; 

 they would then be equally drawn towards that body, and no 

 reason would exist why they should be heaped up under the 

 moon ; for if they were drawn with the same force as that with 

 which the solid globe of the earth under them is drawn, there 

 would be no reason for supposing that the waters would have a 

 greater tendency to collect towards the moon than the solid 

 bottom of the ocean on which they rest. In short, the whole 

 mass of the earth, solid and fluid, being drawn with the same 



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