« AnteriorContinuar »
Philosophers and learned Men as this. Some have thought the Earth and Sea to be a living Creature, which, by it's Respiration, causeth this ebbing arid flowing. Others imagined that it proceeds, and is provoked, from a great Whirl-pool near Norway, which, for six Hours, absorbs the Water, and afterwards disgorges it in the fame space of Time. Scaliger, and others, supposed that it is caused by die opposite Shores, especially of America, whereby the general Motion of the Sea is obstructed and reverberated. But most Philosophers, who have observed the Harmony that these Tides have with the Moon, have given their Opinion, that they are entirely owing to the Influence of that Luminary. But the Question is, what is this Influence? To which they only answer, that it is an occult
Hence also both the Lumi- in Artie.3.)not precisely on the naries, placed in the Equinoctial New and Full Moons, but gene.at the Time of their Conjan- rally they are the third Tides afction or Opposition, which hap- ter them. •- :i pens near the Equinoxes in 6. Things would happen conMarch, or September, produce stantly and regularly thus, if the the highest Tides in the whole whole Earth were covered with Year.' very deep Sea ; but by reason of
Which Experience also con- great and small Wands which
firms, because the Sun is a stop the Tide, and the Streight^
little nearer the Earth in the between them, also the Shelves
Winter than in the Summer; and Shallows along which the
therefore the highest Spring Tides are to be propagated, the
Tides happen a little before Variety of this Phaenomenon is
the Vernal Equinox, and a lit- almost infinite, and scarcely to
tic after the Autumnal, viz. be explained by this Theory;
Jn February and Oclober, rather but when just Observations are
than precisely upon the Bquino- diligently made, all these parti
ilial Days. cular Causes may be found out
■5. The Hbrating Motion of and known. See Newton'*
the Waters, which are apt to Prin. Math. Phil. Book 3. Prop.
retain the Motion impressed up- '24.. Greg. Phys. and Geometr.
on them, and continue to move Astron. Book 4. Prop. 64, 65. al
tho' the Actions of the Lumina- so Halley'j Dissertation in Phil,
lies- cease, make the greatest Trans. No 226.
mnjrualSprini;Tjdes[txphin'd Jurins Appendix,
R 3 Quality, Quality, or Sympathy, whereby the Moon attracts all moist Bodies. But these are only Words, and signify no more than that the Moon does it by some means or other, but they do not know how: Which is the Thing we want.
DES Cartes explains it by his general HypOr thesis thus: In the forementioned Figure of Pro
g)fition 9. let A BCD be the Vortex, with the arth in it's Center, and which, with the Earth and Moon in it, is carried in a larger Vortex about the Sun. Let M be the Center of the first Vortex, EFGH the Earth, 1234 the Superficies of the Sea, which for plainness we will suppose to cover the whole Earth; and 5678 the Superficies of the Air surrounding the Sea. If therefore there were no Moon in the Vortex, the Point T, the Center of the Earth, would coincide with the Point M, the Center of the Vortex; but since the Moon is about B, the Center of the Earth must be between M and D; because, since the celestial Matter of this Vortex moves something swifter than the Earth or Moon, which is carried only with it, unless the Point T were a little further distant from B than from D, the Presence of the Moon would hinder it from moving so freely between B and T, as between T and D •, and seeing the Place of the Earth in the Vortex is not determined, but by the equal Force of the circumambient celestial Matter, it is plain that it ought therefore to apr proach somewhat towards D. And for the fame Reason, when the Moon shall be in C3 the Center of the Earth ought to be between M and A, so that always the Earth may recede a little from the Moon. Moreover, since we supposed the Moon to be about B, not only die Space between B and T, but also that between T and D, thro' both which the celestial Matter flows, is made some-, thing narrower j hence it follows, that the cele'- 2" stial stial Matter floweth faster there, and therefore presseth more, both the Superficies of the Air at o and 8, and of the Water at 2 and 4, than if the Moon had not been in the Diameter of the Vortex BD. Now seeing the Air and Water are both Fluids, and easily give way to the Pressure, they must be more depressed about F and H, than they would be if the Moon were not in this Diameter B D; and also more elevated towards G and E, where both their Superficies bulge or are prominent. And further, because the Part of the Earth at F, under B, where the Sea is now lowest, in six Hours Time will be at G, under C, where it is now highest, and after other six Hours in H, under D, and so on: or rather, because the Moon is moving a little in the mean
whole Revolution A B C D in a Month, by which the part of the Earth that is now in F under the Moon's Body, will be in six Hours, twelve Minutes Time, or thereabouts, a little further than G, in that Diameter pf the Vortex, which is 90 Degr. distant from the Place into which die Moon in the mean Time hath moved; therefore the Water will in that Time increase and be highest at F, and in other six Hours, twelve Minutes, when the Moon is got beyond D, will settle and be lowest there, ($c. Hence it is plain, that the Water of the Sea must constantly ebb and flow in the fame Place, every twelve Hours, twenty four Minutes Time,
THJS js des Cartels Demonstration, which is very ingeniously contrived to account both for the Tides that happen when the Moon is in the Meridian of the Place, and those also that occur when she is in the opposite Point of the Meridian Circk under the Horizon.
Time from B towards C,
BUT according to what we observed in the ninth Proposition, there are several Imperfections in this Demonstration. As first, it is a wonder that des Cartes did not consider, that, according to his Demonstration, the Water ought to ebb at 2 and 4, when the Moon approaches the Meridian B: and, on the contrary, to flow, when the Earth or Moon (viz. either ot them) is removed fix Hours from each other; but this is contrary to Experience, for when the Moon approaches the Meridian of any Place, theWaters flow in that Place, and ebb, back again, at it's departure. • But both des Cartels Words and Figure show the contrary; so that to take away the Absurdity (and in des Cartes's Method) let us suppose the Vortex of the Earth A BCD, and the Waters 1234, to be interspersed equally about the Center T without any Protuberance, and to revolve with the Earth and the celestial Matter between A B C D and 5678. Let us suppose again the Moon to happen into this Vortex at B, and therefore the Space T B to become narrower, and the Water at 2 to be pressed towards E by the celestial Matter squeezing thro' it.
THEN while the Water is expelled from 2 to E, I ask where the greatest swelling will be, whether in the Place E, which is distant a Quadrant from F (where the Moon is vertical), or in the Place next to F towards E? If you answer, the swelling is greatest about the former Place E, I deny it, because it is contrary to Experience j but Experience shews the latter to be true, and even Reason convinces us, that when the Moon is over the Place F, the Water will be forced from 2 towards 1, which happens because the greatest swelling is about 2, not about 1, for here it will be least •, hence the Places to the westward have their Tides later, as we know by Experience. And Reason and the Laws of Hydrostatics require this,'' For if Water be poured in at 2, that it may flow towards E, there will be the greatest quantity of Water at 2,. and a little less in the next Place, but least of all at E; and the fame Thing will happen if it be expelled or driven towards E. But by the Circumrotation of the Earth, E comes into the Place of F, where at length there will be the greatest Protuberance at E, and the Water will be repelled towards H.
THEREFORE^ Cart e's's Figure and Demonstration is to be changed, that the swelling may arise somewhere about 2, viz, where the Moon is vertical. What more might be said here we refer to our Treatise upon des Cartes's Pbyftcs.
The general Motion of the Sea from East to West is 'stronger, and the Tides are higher at New and Full Moon, than at the Quadratures.
THE Truth of this Proposition appears from Experience. For People that use the Sea testify, that at New and Full Moon, the Face of the Ocean is constantly rough and troubled, but calm and quiet at the Quadratures. This is easily accounted for by the aforesaid Hypothesis; for when the Moon is at the New or Full, she is nearer the Earth than at any other Time of her Age, and is furthest distant in her Quadratures, as is shewn by Astronomers (g). But when the Moon is nearer the Earth, that is, when the Space BT is less, the celestial Matter being hindered, or obstructed, presses with greater force the Water from 2 towards 1. But happens 0« therwise in the Quadratures.
(f) This is seise. See the Nate (c) above.