TIDES. 



<kie of the equator with the moon, the day-tide, or that which 

 is produced while the moon is above the horizon of the 

 place, will exceed the night -tide, or that which is produced 

 while the moon is under the horizon of the place. It is the 

 contrary if the moon is on one fide, and the place on the 

 other fide of the equator ; for then the elevation which is 

 oppofite to the moon, moves on the fame fide of the equator 

 with the place, and, therefore, will come nearer to it 

 than the other elevation. The difference will be greateft 

 when the fun and moon both dofcribe the tropics ; becaufe 

 the two elevations in that cafe dcfcribe the oppofite tropics, 

 which are the fartheft from each other of any two parallel 

 circles they can defcribe. Thus it is found, by obfervation, 



world N and S ; but when we conAder that F G H is 

 under the moon's orbit, it will appear that when the moon 

 is over H, in the tropic of Capricorn, the north pole of 

 the tides ( which can be no more than 90° from under the 

 moon) mull be at C in the arftic circle, not at P, the north 

 pole of the earth ; and as the moon afcends from H to G 

 in her orbit, the north pole of the tides mult fliift from 

 c to a in the arftic circle, and the foutli pole as much in 

 the antarftic. 



It is not to be doubted, but that the earth's quick rota- 

 tion brings the poles of the tides nearer to the poles of the 

 worid than they would be if the earth were at reft, and the 

 moon revolved about it only once a month ; for, othenvife, 



that the evening tides in the funimer exceed the morning the tides would be more unequal in their heights, and timea 



tides, and the morning tides in winter exceed the evening of their returns, than we find they are. But how near thf 



tides. The difference is found at Briftol to amount to earth's rotation may bring the poles of its axis and thofe of 



fifteen inches, and at Plymouth to one foot. It would be the tides together, or how far the precedintr tides may 



ftill greater, but that a fluid always retains an impreffed affeft thofe which follow, fo as to make them keep up 



motion for fome tim.e ; fo that the preceding tides affeft nearly to the fame heights, and times of ebbing and flownig, 



always thofe that follow them. Upon the whole, while the is a problem moi-e fit to be folved by obfervation than by 



moon has north declination, the greateft tides in the northern theory 



hemifphere are when (he is above the horizon, and the re- 

 verfe while her declination is fouth. 



To illuftrate this matter by figures ; let N E S Q {Jig. 12, 

 13, 14.) be the earth, NCS its axis, EQ the equator, 



Thofe who have opportunity to make obfervations, and 

 choofe to fatisfy themfelves whether the tides are really 

 affe&ed in the above manner by the different pofitions of the 

 moon, efpecially as to the unequal times of their returns, 



T So the tropic of Cancer, t Vj" the tropic of Capricorn, may take this general rule for knowing when they ought 



ab the arftic circle, cd the antarftic, N the north pole, 

 S the fouth pole, M the moon, F and G the two eminences 

 of water, whofe lowell parts are at a and d (Jig- I2.), at 

 N and S {ftg. 13.), and at b and c {Jig. 14-), always 90^ 

 from the higheft. 



Now, when the moon is in her greateft north declination 

 at M {Jig. 12.), the higheft elevation, G, under her is on 

 the tropic of Cancer, T 25, and the oppofite elevation, F, 

 on the tropic of Capricorn, t Vj" ; and ihefe two elevations 

 defcribe the tropics by the earth's diurnal rotation. All 

 places ;a the northern hemifphere, E N Q, have the 

 liigheft tides when they come into the pofition i s Q, 

 under the moon ; and the loweft tides when the earth's 

 diurnal rotation carries them into the pofition aTE, on the 

 fide oppofite to the moon : the reverfe happens at the 

 fame time in the fouthern hemifphere E S Q, as is evident 

 to fight. The axis of the tides aO. d has now its poles 

 a and d (being always 90° from the higheft elevations) in 

 the ar£lic and antarftic circles ; and, therefore, it is plain, 

 that at thefe circles there is but one tide of flood, and one 

 oi ebb, in the lunar day. For when the point a revolves 

 half round to b in twelve lunar hours, it has a tide of flood ; 

 but when it comes to the fame point a again in twelve 

 hours more, it has the loweft ebb. In fevcn days after 



to be fo affefted. When the earth's "axis inclines' to The 

 moon, the northern tides, if not retarded in their paflage 

 through fhoals and channels, nor affefted by the winds, 

 ought to be greateft when the moon is above the horizon, 

 leaft when ftie is below it ; and quite the reverfe when the 

 earth's axis declines from her ; but in both cafes at equal 

 intervals of time. When the earth's axis inclines fideways 

 to the moon, both tides are equally high, but they happen 

 at unequal intervals of time. In every lunr.tion the earth's 

 axis inclines once to the moon, once from her, and twice 

 fideways to her, as it does to the fun every year ; becaufe 

 the moon goes round the ecliptic every month, and the fun 

 but once in a year. In fummer, the earth's axis inclines 

 towards the moon when new ; and, therefore, the day-tide^ 

 in the north ought to be higheft, and night-tides loweft 

 about the change : at the full the reverfe. At the quarters 

 they ought to be equally high, but unequal in their returns ; 

 becaufe the earth's axis then inclines fide\vife to the moon. 

 In winter the phenomena are the fame at full moon as in 

 fummer at new. In autumn the earth's axis inclines fide- 

 wife to the moon when new and full ; therefore the tides 

 ought to be equally high and unequal in their returns at 

 thefe times. At the firft quarter the tides of flood fnould 

 be leaft when the moon is above the horizon, greateft when 



ward, the moon M {Jig. 13.) comes to the equinoftial file is belovv' it, and the reverfe at her third quarter. In 



circle, and is over the equator E Q, when both elevations 

 defcribe the equator ; and in both hemifphercs, at equal 

 diftances from tlie equator, the tides are equally high in 

 both parts of the lunar day. All the phenomena being 

 reverfed, when the moon has fouth declination, to what they 

 were when her declination was north, require no farther 

 delcription,^^^. 14. 



fpring, the phenomena of the firft quarter anfwer to thofe 

 of the third quarter in autumn, and vice vcrfd. The nearer 

 any time is to either of thefe feafons, the more the tides 

 partake of the phenomena of thefe feafons ; and in the 

 middle between any two of them, the tides are at a mean 

 ftate between thofe of both. 



8. Such would the tides regularly be, if the earth were 



From what has been faid it appears, that as the tides are all over covered with fea very deep, fo that the water might 



governed by the moon, they muft turn on the axis of the follow the influence of the fun and moon ; but, by reafon of 



moon's orbit, which is inclined 23^ degrees to the earth's the fhoalnefs of fome places, and the narrownefs of the 



axis at a mean ftate ; and, therefore, the poles of the tides ftreights in others, by which the tides are propagated, there 



muft be fo many degrees from the poles of the earth, or in aiifes a great diverfity in the effcft, not to be accounted for, 



oppofite points of the polar circles, going round thefe without an exaft knowledge of all the circumftances of the 



circles in every lunar day. It is true that, according to places; fuch as the pofition of the land, and the breadth 



.Af- '4> when the moon is vertical to the equator E C Q, and depth of the channels, diredtion of the winds, &c. For 



the poles of the tides fccm to fell in v.ith the poles of the a very flow and imperceptible motion of the whole body of 



water, 



