CHAPTER II.. Problems of Tides.

PROBLEMS OF TIDES.

A. What makes the flux and reflux of the sea, twice in a natural day?

B. We must come again to our basin of water; wherein you have seen, whilst it was moved, how the water mounteth up by the sides, and withal goes circling round about. Now if you should fasten to the inside of the basin some bar from the bottom to the top, you would see the water, instead of going on, go back again from that bar ebbing, and the water on the other side of the bar to do the same, but in counter-time; and consequently to be highest where the contrary streams meet together; and then return again, marking out four quarters of the vessel; two by their meeting, which are the high waters; and two by their retiring, which are the low waters.

A. What bar is that you find in the ocean that stops the current of the water, like that you make in the basin?

B. You know that the main ocean lies east and west, between India and the coast of America; and again on the other side, between America and India. If therefore the earth have such a motion as I have supposed, it must needs carry the current of the sea east and west: in which course, the bar that stoppeth it, is the south part of America, which leaves no passage for the water but the narrow strait of Magellan. The tide rises therefore upon the coast of America; and the rising of the same in this part of the world, proceedeth from the swelling chiefly of the water there, and partly also from the North Sea; which lieth also east and west, and has a passage out of the South Sea by the strait of Anian, between America and Asia.

A. Does not the Mediterranean Sea lie also east and west? Why are there not the like tides there?

B. So there are, proportionable to their lengths and quantity of water.

A. At Genoa, at Ancona, there are none at all, or not sensible.

B. At Venice there are, and in the bottom of the straits, and a current all along both the Mediterranean Sea and the Gulf of Venice: and it is the current that makes the tides insensible at the sides; but the check makes them visible at the bottom.

A. How comes it about that the moon hath such a stroke in the business, as so sensibly to increase the tides at full and change?

B. The motion I have hitherto supposed but in the earth, I suppose also in the moon, and in all those great bodies that hang in the air constantly, I mean the stars, both fixed and errant. And for the sun and moon, I suppose the poles of their motion to be the poles of the equinoxial. Which supposed, it will follow (because the sun, the earth, and the moon, at every full and change are almost in one straight line) that this motion of the earth will then be made swifter than in the quarters. For this motion of the sun and moon being communicated to the earth, that hath already the like motion, maketh the same greater; and much greater when they are all three in one straight line, which is only at the full and change, whose tides are therefore called spring tides.

A. But what then is the cause that the spring tides themselves are twice a-year, namely, when the sun is in the equinoxial, greater than at any other times?

B. At other times of the year, the earth being out of the equinoxial, the motion thereof, by which the tides are made, will be less augmented, by so much as a motion in the obliquity of twenty-three degrees, or thereabout, which is the distance between the equinoxial and ecliptic circles, is weaker than the motion which is without obliquity.

A. All this is reasonable enough, if it be possible that such motions as you suppose in these bodies, be really there. But that is a thing I have some reason to doubt of. For the throwing off of air, consequent to these motions, is the cause, you say, that other things come to the earth; and therefore the like motions in the sun and moon and stars, casting off the air, should also cause all other things to come to every one of them. From whence it will follow, that the sun, moon, and earth, and all other bodies but air, should presently come together into one heap.

B. That does not follow. For if two bodies cast off the air, the motion of that air will be repressed both ways, and diverted into a course towards the poles on both sides; and then the two bodies cannot possibly come together.

A. It is true. And besides, this driving of the air on both sides, north and south, makes the like motion of air there also. And this may answer the question, how a stone could fall to the earth under the poles of the ecliptic, by the only casting off of air?

B. It follows from hence, that there is a certain and determinate distance of one of these bodies, the stars, from another, without any very sensible variation.

A. All this is probable enough, if it be true that there is no vacuum, no place empty in all the world. And supposing this motion of the sun and moon to be in the plain of the equinoxial, methinks that this should be the cause of the diurnal motion of the earth; and because this motion of the earth is, you say, in the plain of the equinoxial, the same should cause also a motion in the moon on her own centre, answerable to the diurnal motion of the earth.

B. Why not? What else can you think makes the diurnal motion of the earth but the sun? And for the moon, if it did not turn upon its own centre, we should see sometimes one, sometimes another face of the moon, which we do not.

==========