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being the shape and position which most checks and obstructs the motion from east to west; I conceive that the waters would hold on in their straight and natural course for a time, but that afterwards, being driven back by that island, they would return in equal intervals; so that there would only be one flow and one ebb in the course of the day, and about twelve hours would be given to each of them. And now suppose (what is indeed the fact) the earth to be divided into two islands, namely, the Old and New World (for the southern continent from its position does not make much difference, as neither do Greenland or Nova Zembla), and these two islands to extend almost through three zones, between which the two oceans, the Atlantic and Southern, flow, but have no passage through except towards the poles ; I conceive it must needs follow that these two obstacles will infuse and communicate the nature of a twofold reciprocation to the whole body of the water, and thence comes that quarter of the diurnal motion ; for that the waters being checked on both sides, the ebb and flow of the sea must come twice a day, every six hours, there being a double advance and likewise a double repercussion. And if these two islands were extended in the waters like cylinders or columns, with equal dimensions and straight shores, this motion, which now seems confused and obscured by reason of the variety of position in sea and land, would be easily demonstrated, and would suggest itself to anybody. Neither likewise is it difficult to form some conjecture of the degree of velocity that
be reasonably assigned to this motion of the waters, and of the distance it performs in one day. For if (to estimate this) you take some of those shores which are
least mountainous or depressed, and are contiguous to an open sea, and if you take a measure of the distance between high and low water mark, and if you multiply this distance by four on account of the four tides a day, and again double the product on account of the tides at the opposite shores of the same sea, and add something more on account of the height of coasts, which are always somewhat raised above the level of the sea ; this calculation will give the distance which a globe of water, if it were free from all obstruction and always moved in a circular progression round the earth, would travel ; and certainly it is not a great one. Now with respect to that difference which coincides with the moon's motion, and makes the period a month ; I conceive it to be due to this : that the time of six hours is not the exact measure of reciprocation, as neither is the diurnal motion of any of the planets restored exactly in twenty-four hours; and that of the moon least of all. Therefore the measure of the ebb and flow of the sea is not a quarter of the motion of the fixed stars, which is the motion of twenty-four hours, but a quarter of the diurnal motion
Injunctions. Inquire whether the time of high water about the coast of Africa precedes that about the Straits of Gibraltar. Inquire whether the time of high water abcut Norway precedes that of high water about Sweden; and in like manner whether the latter precedes that about Gravelines.
Inquire whether the time of high water on the coast of Brazil precedes the time of high water on the coasts of New Spain and Florida.
Inquire whether the time of high water on the coast of China is not the same, or very nearly the same, as the time of high water on the coast of Peru; and also as the time of low water on the coasts of Africa and Florida.
Inquire how the time of high water on the coast of Peru differs from that of high water on the coast of New Spain, and particularly of the differences in the hours of high water on the two shores of the Isthmus of Darien ; and again how the time of high water on the coast of Peru corresponds to the time of high water on the coast of China.
Inquire of the heights of the tides on different coasts, as well as of their times and hours. For though high tides are mostly caused by depressions of coasts, yet they have some relation likewise to the true motion of the sea, according as it is with them or against them.
Inquire of the Caspian Sea (which is a large landlocked collection of waters, with no outlet to the ocean) to see if it has any ebb and flow, or of what nature it is; for my own conjecture is, that the waters of the Caspian may have one tide a day, but not two; and such that there shall be low water on the eastern coasts of that sea, when there is high water on the western.
Inquire whether the higher flood tides at the new and full moon, and likewise at the equinoxes, take place in different parts of the world at the same time; and when I say at the same time, I do not mean the same hour (for the hours vary, as I have said, according to the progression of the waters along the shore), but the same day.
340 TRANSL. OF "DE FLUXU ET REFLUXU MARIS.”
Limitations. The inquiry is not carried out to a full explanation of the correspondence of the monthly motion of the sea with the motion of the moon; as to whether it be the effect of subordination, or of a common cause.
The present inquiry is connected with the inquiry whether the earth has a diurnal motion. For if the tide be, as it were, the extreme diminution of the diurnal motion, it will follow that the globe of the earth is immovable, or at least that it moves much slower than the waters themselves.