| XXX | (475) | XXX |
475 ASTRO N O M V. pics by the earth’s diurnal rotation. All places in the And thus it appears, tha£ as the tides are governed northern hemifphere EN§>~ have the higheft tides when by the moon, they muft turn on the axis of the moon’s they come into the pofition bSB under the moon ; orbit, which is inclined 234 degrees to the earth’s axis and the loweft tides when the earth’s diurnal rotation at a mean ftate : and therefore the poles of the tides muft carries them into the pofition aTE, on the fide oppofite be fo many degrees from the poles of the earth, or in to the moon ; the reverfe happens at the fame time in oppofite points of the polar circles, going round thefe the fouthern hemifphere ESQ_, as is evident to fight. circles in every lunar day. It is true, that, according to The axis of the tides aCd has now its poles a and d '(be- Plate XLV. fig. 4. when the moon is vertical to the ing always 90 degrees from the higheft elevations) in the equator EC^, the poles of the tides feem to fall in with ardtic and antardlic circles ; and therefore it is plain, the poles of the world N and S : but when we confider that at thefe circles there is but one tide of flood, and that FHG is under the moon’s orbit, it will appear, that ' one of ebb, in the lunar day. For, when the point a when the moon is over i/, in the tropic of Capricorn, revolves half round to b, in 12 lunar hours, it has a tide the north pole of the tides (which can be no more than of flood; but when it comes to the fame point a again 90 degrees from under the moon) muft be at c in the in 12 hours more, it has the loweft ebb. In feven days arClic circle, not at N, the north pole of the earth ; afterward, the moon M comes to the equinoctial circle, and as the moon afeends from i/ to (7 in her orbit, the and is over the equator 'when both elevations de- north pole of the tides muft fhift from c to ^ in the fcribe the equator ; and in both hemifpheres, at equal arftic circle, and the fouth pole as much in the antanftic. diftances from the equator, the tides are equally high in It is not to be doubted, but that the earth’s quick both parts of the lunar day. The whole phenomena be- rotation brings the poles of the tides nearer to the poles ing reverfed, when the moon has fouth declination, to of the world, than they would be if the earth were at what they were when her declination was north, require reft, and the moon revolved about it only once a month ;, no farther defcription. for otherwife the tides would be more unequal in their In Plate XLV. fig. 3, 4. 5. the earth is ortho- heights, and times of their returns, than we find they graphically projedted on the plane of the meridian ; but are. But how near the earth’s rotation may bring the in order to defcribe a particular phenomenon, we now poles of its axis and thofe of the tides together, or how projedt it on the plane of the ecliptic. In the fame Plate far the preceding tides may affedt thofe which follow, fo fig. 2. let HZQN be the earth and fea, FED the equa- as to make them keep up nearly to the fame heights, and tor, T the tropic of Cancer, C the ardtic circle, P the times of ebbing and flowing, is a problem more fit to be north pole, and the curves 1 2 24 meridians, or fblved by obfervation than by theory. hour-circles, interfedling each other in the poles; ACM Thofe who have opportunity to make obfervations,. is the moon’s orbit, S the Sun, M the moon, Z the and chufe to fatisfy themfelves whether the tides are water elevated under the moon, and the oppofite eaff&fied in the above manner by the different pofiqual elevation. As the loweft: parts of the water are al- really of the moon, efpecially as to the unequal times of ways 90 degrees from the higheft, when the moon is in tions their returns, this general rule for knowing either of the tropics, (as at /If), the elevation Z is on when they oughtmaytotake be fo affe&ed. When the earth’s the tropic of Capricorn, and the oppofite elevation N on axis inclines to the moon, the northern, tides, if not rethe tropic of Cancer, the low-water circle HCO touches tarded in their paflage through and channels, nor afthe polar circles at C and the high-water circle ETP6 fedted by the winds, ought to fhoals when the moon • goes over the poles at P, and divides every parallel of is above the horizon, leaft whenbe{hegreateft is below it, and quite latitude into two equal fegments. In this cafe the tides the reverfe when the earth’s axis declines from her ; but, upqj| every parallel are alternately higher and lower ; but cafes, at equal intervals of time. - When the earth’s they return in equal times : The point T, for example, inaxisbothinclines fidewife to the moon, both tides are equally on the tropic of Cancer, (where the depth of the tide is: but they happen at unequal intervals of time. In . reprefented by the breadth of the dark ftiade), has a high, every lunation the earth’s axis incliaes once to the moon, fhallower tide of flood at T than when it revolves half, once from her; and twice fidewife to her, as it does round from thence to, 6-, according to the order of the the fun every year ;, becaufe the moon goes round theto > numeral figures ; but it revolves as foon from 6 to 7" as ecliptic every month, and the fun but once in a year. it did from T to 6. When the moon is in the equi- fummer, the earth’s axis inclines towa.rds the moon noctial, the elevations Z and N are transferred to the Inwhen, and. therefore the day-tidCs in the north equator at 0 and and the high and low-water circles ought new; . and night-tides loweft about the are got into each other’s former places ; in which cafe change to; atbethehigheft, full the reverfe. At the quarters they the tides return in unequal times, but are equally high in ought to be equally high, but unequal in their returns ; both parts of the lunar day : for a place at 1 (under D) becaufe the earth’s axis then fidewife to the revolving as formerly, goes fooner from 1 to 11, (un- moon. In winter the phenomenainclines are the fame at fullder F),. than from 11 to 1, becaufe the parallel it de- moon as in fummer at new. In autumn earth’s axis feribes is cut into unequal fegments by the high-water inclines fidewife to, the moon-when new andthe full; therecircle HCO •. but the points 1 and ix being equidiftant fore the tides ought to be equally high,. and unequal from the pole of the tides at C, which is direCHy under their returns at thefe times. At the firft quarter thein the pole of the moon’s orbit MCAA, the elevations are e- tides of flood ftiould be leaft when the moon is above the qualiy high in both parts of the day. horizon, greateft when ftie is below it; and the.reverfe
