a Correfpondence between the Motion of the Sea, and that of the Moon, for when this approaches the other, it causes it to fwell at 2, and the Currents are obferved to fet ftronger to the weftward at the New and Full Moon, than at the Quadratures. This laft is excellently explained by des Cartes's Method; for fince the Moon is nearer the Earth at the New or Full than when she is in the Quadratures, the Paffage for the celeftial Matter, B6, is then made narrower, and therefore the Preffure is greater (e). IF any fhould alledge, that perhaps the greater Light of the Moon, at Full, causes the greater Intumefcence; I anfwer, that at the Change all her Light is taken away; which fhews that Light is not the Cause of this Motion, but rather that Preffure of des Cartes, which we fhall further explain below. PROPOSITION X. The fecond general Motion of the Sea is it's Flux and Reflux, by which, in about twelve Hours and a balf's Time, the Water is found to flow towards the generality of Shores, and to ebb back again, viz. to flow when the Moon approaches the Meridian Circle above or below; and to ebb when it departs from thence towards the Horizon. WE are first to enquire whether the Sea flows towards one certain Point by this Motion, viz. from East to West, or from West to East. FOR the Shores of Bays, and the Chanels. of Rivers, where this Flux and Reflux is chiefly obferved, more than in the main Ocean, are divers ways extended; fome from Weft to East, as the Mediterranean Sea, and others from South to North, as the Arabian Gulph, &c. And in all these the Water flows thro' the Streights towards the furtheft Point of their extent; and therefore in different Bays, this Flux of the Ocean tends towards divers Points of the Compafs. We must therefore first be refolved, whether this Flux, or Motion, tends indifferently to any Point, or only obferves two, viz. the Weft in flowing, and the Eaft in ebbing; or even only the Weft in both ebbing and flowing? In my Opinion the last is truest, viz. that the whole Ocean is moved from Eaft to Weft, both in it's Flux and Reflux, and that the difference is, that in it's Flux it is 'moved with greater violence and in a greater Quantity: but in it's Reflux (or more properly it's Deflux) tho' it be not moved a contrary Way, yet it seems to be fo, because there flows a lefs Quantity of Water. HENCE we may determine, that the Flux and Reflux of the Sea is no way diftinct from that general Motion, which we explained in the former Propofition, whereby the Ocean is perpetually moved from East to Weft; for it is only a certain Mode or Property of that Motion. And therefore if this Motion be obferved, and rightly confidered in the main Ocean, where it is not obftructed, we fhall find it not to be fo much a Flux and Reflux of the Sea, as a Flux and Deflux, or (that we may distinguish, by proper Terms, the Quality of the Motion or Flux from the MoVOL. I. R tion tion or Flux itself), it is most aptly called the Swelling and Swaging of the Sea. FOR the Sea perpetually flows from Eaft to Weft, and only feems to flow back again, when it's more violent Force is flackened and wafted, which a little before was quickened and augmented. But this is called the Reflux, because the Sea feems, on Shores and in Bays, to approach and retire by fits, which is not owing to the quality of the Motion itself, but to the Situation of the Shores and Bays, which requires that the Water fhould fall back to the contrary Point; but the fettling of the Sea in general doth not proceed from the Situation of the Shores, but from the quality of the Motion of the Water. BUT the Motion of the Sea can by no means be estimated by it's approach to the Shores, for whatever this Motion be, or to what Point foever it is made, it will always fluctuate towards the Shores; which happens by reafon of the fluid Nature of the Water. THAT the Sea moves towards the fame Point, that is, from Eaft to Weft both in the Flux and Reflux (or Swelling and Swaging) and never moves the contrary way appears from the following Obfervations. 1. In the main Ocean between the Tropics, there is no other Motion perceived than this from East to Weft. 2. In Streights that join the Parts of the Ocean and run directly Eaft and Weft, as the Streights of Magellan, Manilba, Java, and others among the Indian Islands; in thefe, I fay, the Sea rises and settles in 12 Hours Time, but in fettling it doth not flow back out of the Streights to the eastward; but is carried by other Paffages, ftill to the weftward which is a plain Sign that this Ebbing and Flowing are not two contrary Motions, but a Modification of the general Motion from East to West. So So that Scaliger and all the reft are deceived, who represent this as a double Motion to and again. IT is to be understood, that when we fay this Motion is from Eaft to Weft, we do not mean punctually the two cardinal Points, but include all their Collaterals, even to the North and South Poles, towards which however the Motion is weaker. PROPOSITION XI. To explain the Caufe of the Swelling and Swaging of the Sea, vulgarly called it's Flux and Reflux (f). (f) Sir Ifaac Newton moft fuccefsfully explains as well the Flux and Reflux of the Sea, as moft other Appearances of Nature, from his univerfal Principle of Gravity or Attraction. Gravitation is a certain Force imprinted on all Bodies by the Author of Nature, by which they mutually endeavour to accede; but how this Force is exerted we know not. Thus the Globe of the Sun and Planets gravitate mutually towards each other in proportion to their feveral Maghitudes, and Distances from one another. As to this Earth of ours, it hath but little Communication with the other Planets, whofe Bodies are too fmall to affect us much, at fuch a vaft Distance; only the Sun and Moon are respected by it, the one because it is placed fo near us, and the other by reafon of the Bulk of it's Body; which tho' it be at a vast Distance, yet acts with a strong attractive Force. For a Body is more THERE forcibly attracted by how much the Distance of the Attrahent is nearer, or it's Bulk greater. 1. Thus; Let L (Fig. 23.) be the Moon, fuppofed to be above any Part of the Earth, covered with the Ocean as b; it is evident that this Place, being nearer the Moon than any other Part of the Earth, is more ftrongly drawn thereby, and fwelleth up towards it: But the Water in the Place a being diametrically oppofite to the Place b, and further off from the Moon than the rest of the Earth, hath a lefs Tendency towards it than the other Parts; and therefore, being left as it were by the Earth, is lifted up, or fwelled, the contrary Way in a. Hence the Water flowing from d to e towards A and B, makes two Protuberances in the Ocean, the one in B directly under the Moon, the other in A juft oppofite to it; and these always fhift and accompany the Moon in it's feeming Motion R 2 about THERE is no Phænomenon in Nature that hath fo much exercised and puzzled the Wits of Philofo about the Earth, and occafion thereby two Floods and Ebbs in the fame Place, every five and twenty Hours. 2. Of these two Tides that happen in the Time of one diurnal Revolution in any Place, that is the greatest, wherein the Place cometh nearest the Eminence of the Water A or B. Thus, in fuch a Figure as the laf, let, Pp (Fig 24.) be the Poles, Q the Equator, FG a Parallel to it, which any place defcribes by it's diurnal Motion; it appears that the two High waters happen in the Place, when it is fituated in G or F, having the Moon in the Meridian; but the highest Tide is found in the Point G, which comes nearest the Eminence of the Waters in B. It further appears from the Figure, that the Moon, in the Time of the highest Tide, is above the Horizon of the Place, if she is on the fame fide of the Equator with the Place itself: but if the decline the contrary Way, she is under the Horizon in the Point A, at the Time of the higheft Tide. For Example, in Europe the diurnal Tides are the highest of the two when the Moon is found in the elevated Semicircle of the Meridian, or in the Northern Signs of the Ecliptic; but the lowest when she is in the Southern Signs. Moreover, the Height of the Tides is varied generally all over the Earth, according to the day I of the Month and the time of the Year. 3. For, because the attractive Force of the Sun reaches the Earth as well as that of the Moon; when both thefe Forces confpire, or are united, they raise the Waters higheft, and make what we call Spring Tides; but when the Sun depreffes what the Moon heaves up, then happen the loweft or Neap Tides. Thus we obferve higher Tides when the Sun and Moon in Conjunction or Opposition, are right over any Place B, or diametrically oppofite over A and B, than when they are in the Quadratures, viz. when the Sun is in the Point H or I, and the Moon in the intermediate Point A or B. But the Force of the Sun is fmall compared with that of the Moon; becaufe the Semidia-. meter of the Earth CB, by which the Water in B is nearer the Sun than the Center C, is fcarce fenfible, if compared with the immense Distance of the Sun. 4. Since the Eminences of Water are carried round the Earth by the diurnal Motion, the Motion, Agitation, and Height, of the Tides, are the greater, the larger the Circle is in which the Waters revolve. So the Moon being in the Equino&tial, and leading about the two oppofite Eminences of Water in the Equator, makes greater Tides (cæteris paribus) than when the is in the Tropics. Hence |