particles of water on the side of the Earth turned to it, all of which tend to close up under the Moon. The force acting upon these particles is called the tangential component of the attraction; and this is by far the most powerful cause of the tides, as it acts at right angles to the Earth's gravity, whereas the direct attraction of the Moon acts in opposition to it.

665. The spring and neap tides, which, as we have seen, depend upon the combined or opposed action of the Sun and Moon in longitude, are also influenced by the difference of latitude between the two bodies. Of course, that spring tide will be highest which occurs when the Moon is nearest her node, or in the ecliptic. The apex of the semi-diurnal tide also follows the Moon throughout her various declinations.

666. The phenomena of the tides are greatly complicated by the irregular distribution of land. The time of high water at any one place occurs at the same interval from the Moon's passage over the meridian; this period is different for different places. The interval at new or full Moon between the times of the Moon's meridian passage and high water is termed the establishment of the port.

667. Although in the open ocean the velocity of the tidal undulation may be 500 or even 900 miles an hour, in shallow waters the undulation is retarded to even seven miles at the same time that its height is increased. The average height of the tide round the islands in the Atlantic and Pacific Oceans is but 3 feet; whereas at the head of the Bay of Fundy it is 70 feet. As the tidal undulation does not move so rapidly as the Earth does, since it is regulated by the Moon, it appears to move westward while the Earth is moving eastward; and it has been suggested that this apparent backward movement acts as a break on the Earth's rotation, and that, owing to the effects of tidal action, the diurnal rotation is, and has been, constantly decreasing in velocity to an extremely minute extent. At

all events, if the sidereal day be assumed to be invariable, it is impossible to represent the Moon's true place at intervals 2,000 years apart by the theory of gravitation. On this assumption the Moon, looked upon as a timepiece, is too fast by 6" or 12s. (nearly) at the end of each century. This may be due to the fact that our standard of measurement of the sidereal day is too slow; and it has been calculated that this part of the apparent acceleration of the Moon's mean motion may be accounted for by supposing that the sidereal day is shortening, in consequence of tidal action, at the rate of th part of a second in 2,500 years.

The End.

APPENDIX.

TABLE I. Astronomical Symbols and Abbreviations.

II. Elements of the Planets.

VII. Conversion of Intervals of Sidereal Time into Mean Time.