we are not yet in a position to lay down the laws which govern it; if the whole globe were uniformly covered, the velocity would be rather more than 1000 miles per hour (7926 × 31416÷248). It is probably, however, nowhere equal to this, unless perhaps in the Antarctic Ocean. The following table of velocities is given by Whewell :

Concerning the general character of the great terrestrial tidal wave, I cannot do better than quote the following description by a well-known eminent geographer :

"The Antarctic is the cradle of tides. It is here that the Sun and Moon have presided over their birth, and it is here, also, that they are, so to speak, to attend on the guidance of their own congenital tendencies. The luminaries continue to travel round the Earth (apparently) from East to West. The tides no longer follow them. The Atlantic, for example, opens to them a long, deep canal, running from North to South, and after the great tidal elevation has entered the mouth of this Atlantic canal, it moves continually Northward; for the second 12 hours of its life it travels north from the Cape of Good Hope and Cape Horn, and at the end of the first 24 hours of its existence, has brought high water to Cape Blanco on the West of Africa, and Newfoundland on the American continent. Turning now round to the Eastward, and at right angles to its original direction, this great tidal wave brings high water, during the morning of the 2nd day, to the Western coasts of Ireland and England. Passing round the Northern cape of Scotland, it reaches Aberdeen at noon, bringing high water also to the opposite coasts of Norway and Denmark. It has now been travelling precisely in the opposite direction to that of its genesis, and in the opposite direction, also, to the relative motion of the Sun and Moon. But its erratic course is not yet complete. It is now travelling from the Northern mouth of the German Ocean Southwards. At midnight of the 2nd day it is at the mouth of the Thames, and wafts the merchandise of the world to the quays of the port of London. In the course of this rapid journey the reader will have noticed how the lines [on the map] in some parts are crowded together closely on each other, while in others they are wide asunder. This indicates that the tide-wave is travelling with varying velocity. Across the southern ocean it seems to travel nearly 1000 miles an hour, and through the Atlantic scarcely less; but near some of the shores, as on the coast of India, as on the East of Cape Horn, as round the shores of Great Britain, it travels very slowly; so that it takes more time to go from Aberdeen to London b Phil. Trans., vol. exxiii. p. 212. 1833.

than over the arc of 120° which reaches from 60° of Southern latitude to 60° North of the Equator. These differences have still to be accounted for; and the high velocities are invariably found to exist where the water is deep, while the low velocities occur in shallow water. We must therefore look to the conformation of the shores and bottom of the sea as an important element in the phenomena of the tides "."

The effects of tides on rivers are often very striking; especially is this the case with the Avon at Bristol: when the tide is at its ebb, the river is little better than a shallow ditch, but when the waters have risen to the maximum height, an insignificant stream is converted into a broad and deep channel, navigable by the largest Indiaman.

The instinct of animals in respect of the tides is often very remarkable. A Scotch writer observes: "The accuracy with which cattle calculate the times of ebb and flow, and follow the diurnal variations, is such, that they are seldom mistaken, even when they have many miles to walk to the beach. In the same way they always secure their retreat from these insulated spots in such a manner that they are never surprised and drowned d."

In their passage up rivers, tides are gradually extinguished, as will be seen from the following table relating to the Thames " :—

At certain places on the coast of Hampshire and Dorsetshire the waters of the ocean ebb and flow twice in 12 hours instead of only once, as is usual elsewhere. Southampton, Christchurch, Poole, Weymouth, and the Firth of Forth, may be mentioned as places where this singular phenomenon has been observed f.

Another abnormal tidal phenomenon, presenting some remarkable features, occurs once a year in the rivers Severn, Humbers, and Loire, and in some other rivers of the same character as regards the formation of their banks. This is the "hygre," or "bore," and is due to the fact that a wide estuary at the mouth of the river

suddenly contracts like a funnel. The result is, that the estual spring tide rushes up with an overpowering force, carrying all before it. This further peculiarity likewise subsists: namely, that there is no "slack-water," as is ordinarily the case in other rivers, between the ebb and flow of the tide. The approach of the bore on the Severn may be heard at a considerable distance roaring, as it were, in its upward progress. The head is about 3 feet high, and it frequently does a good deal of mischief to property. The maximum effect is at the 4th tide after the Full Moon.

The evident connexion between the periods of the tides and those of the phases of the Moon led to the tides being attributed to the Moon's action long before their true theory was understood. Aristotle and Pytheas of Marseilles are both said to have pointed out the connexion. Julius Cæsar adverts to the connexion existing between the Moon and spring tides1.

n

Pliny says: "Estus maris accedere et reciprocare, maxime mirum: pluribus quidem modis: verum causa in sole lunáquem" Kepler clearly indicated that the principle of gravitation is concerned an opinion from which Galileo strongly dissented o. Wallis, in 1666, also published a tidal theory". Before Sir Isaac Newton turned his attention to this subject, the explanations given were at best but vague surmises. "To him was reserved the glory of discovering the true theory of these most remarkable phenomena, and of tracing, in all its details, the operation of the cause which produces them."

CHAPTER III.

PHYSICAL PHENOMENA.

Secular Variation in the Obliquity of the Ecliptic.-Precession.-Its value.-Its physical cause.-Correction for Precession.-History of its discovery.—Nutation. -Herschel's definition of it.-Connexion between Precession and Nutation.

NECULAR Variation in the Obliquity of the Ecliptic.- Although

SECUL

it is sufficiently near for most purposes to consider the inclination of the plane of the ecliptic to that of the equator as invariable, yet this is not strictly the case, inasmuch as it is subject to a small but appreciable change of 46′45′′ (C. A. F. Peters) per century. This phenomenon has long been known to astronomers, on account of the increase it causes in the latitude of all stars in some situations, and corresponding decrease in the opposite regions. Its effect at the present time is to diminish the inclination of the two planes of the equator and the ecliptic to each other; but this diminution will not go on a beyond certain very moderate limits, after which it will again increase, and thus oscillate backwards and forwards through an arc of 1° 21′ the time occupied in one oscillation being about 10,000 years. One effect of this variation of the plane of the ecliptic-that which causes its nodes on a fixed plane to change-is associated with the phenomena of the precession of the equinoxes, and cannot be distinguished from it, except in theory b.

:

Precession. The precession of the equinoxes is a slow but con

a Compare Genesis viii. 22.

The inclination of the ecliptic for

the epoch of January 1, 1878, is 23° 27′ 18.50".

tinual shifting of the equinoctial points from East to West. Celestial longitudes and right ascensions are reckoned from the vernal equinox, and if this were a fixed point, the longitude of a star would never vary, but would remain the same from age to age as does its latitude (sensibly). Such, however, is not the case; as it has been found that apparently all the stars have changed their places since the first observations were made by the astronomers of antiquity. Two explanations only can be given to account for this phenomenon: we must either suppose that the whole firmament has advanced, or that the equinoctial points have receded. And as these points depend on the Earth's motion, it is far more reasonable to suppose that the phenomenon is owing to some perturbation of our globe rather than that the starry heavens should have a real motion relative to these points. The latter explanation is accordingly adopted, namely, that the equinoxes have a periodical retrograde motion from East to West, thereby causing the Sun to arrive at them sooner than it otherwise would had these points remained stationary. The annual amount of this motion is, however, exceedingly small, being only equal to 50.2"; and since the circle of the ecliptic is divided into 360°, it follows that the time occupied by the equinoctial points in making a complete revolution of the heavens is 25,817 years. It is owing to precession that the Pole-star varies from age to age, and also that whilst the sidereal year, or actual revolution of the Earth round the Sun, is 365d 6h 9m 110, the equinoctial, solar, or tropical year is only 365d 5h 48m 46.05 (Airy). The successive

• It may be well to mention that the equinoxes are the two points where the ecliptic cuts the equator; and so called because when the Sun in its annual course arrives at either of them, day and night are equal throughout the world. The point where the Sun crosses the equator, going north, is known as the vernal equinox; and the opposite point, through which the Sun passes going south, as the autumnal equinox. These intersecting points are also termel nodes, and an imaginary line joining the two, the line of nodes. The ascending node (8) answers to the vernal equinox, and the descending (8) to the autumnal.

d By "change of place" is here meant change of position of the Sphere as a

whole to certain fixed co-ordinates, not change of place of the Stars inter se, so as to alter the figures of the Constellations; although many individual stars-as we shall see hereafter-have very considerable proper motions.

Bessel, by a careful discussion of the most reliable observations, fixed the value of general precession for the epoch of 1750 at 50 21129", and the value of lunisolar precession at 5037572". For the epoch of 1800 he gave for the value of the latter 50 36354". The lunar precession is about 2 times the solar precession, just as the lunar tide is 2 times the solar tide, and for much the same reason, namely, the difference of the attractions.