| Fig. 230. | Fig. 231. |
WHY HIGH TIDE VARIES DAILY.
The moon travels round the earth once in twenty-eight days. In Fig. 231 the point a is nearest the moon at, say, twelve noon. At the end of twenty-four hours it will have arrived at the same position by the compass, but yet not be nearest to the moon, which has in that period moved on 1⁄28th of a revolution round the earth.[43] Consequently high tide will not occur till a has reached position b and overtaken the moon, as it were, which takes about an hour on the average. This explains why high tide occurs at intervals of more than twelve hours.
|
Fig. 232.—Relative positions of sun, moon, and earth at "spring" tides. |
Fig. 233.—Relative positions of sun, moon, and earth at "neap" tides. |
NEAP TIDES AND SPRING TIDES.
The sun, as well as the moon, attracts the ocean, but with less power, owing to its being so much further away. At certain periods of the month, sun, earth, and moon are all in line. Sun and moon then pull together, and we get the highest, or spring tides (Fig. 232). When sun and moon pull at right angles to one another—namely, at the first and third quarters—the excrescence caused by the moon is flattened (Fig. 233), and we get the lowest, or neap tides.
[39] In both Figs. 207 and 208 the degree of expansion is very greatly exaggerated.
[40] As the sun passes the meridian (twelve o'clock, noon) the chronometer's reading is taken, and the longitude, or distance east or west of Greenwich, is reckoned by the difference in time between local noon and that of the chronometer.
[41] For much of the information given here about clocks and watches the author is indebted to "The History of Watches," by Mr. J.F. Kendal.
[42] We shall here notice only those gears which are included in the hub of the driving-wheel.