SECONDARY CAUSE OF HIGH TIDE OPPOSITE THE MOON.

1. The point A represents the center of gravity between the earth and moon; and as it is this point which traces the regular curve of the earth's orbit, it is represented in the arc of that orbit, while the earth's center is 6,000 miles one side of it. Now the law of gravitation requires that while both the moon and earth revolve around the sun, they should also revolve around the common center of gravity between them, or around the point A. This would give the earth a third revolution, in addition to that around the sun and on her axis. The small circles show her path around the center of gravity, and the arrows her direction.

2. This motion of the earth would slightly increase the centrifugal tendency at B, and thus help to raise the tide-wave opposite the moon. But as this motion is slow, corresponding with the revolution of the moon around the earth, the centrifugal force could not be greatly augmented by such a cause.

283. As the moon, which is the principal cause of the tides, is revolving eastward, and comes to the meridian. later and later every night, so the tides are about 50 minutes later each successive day. This makes the interval between two successive high tides 12 hours and 25 minutes. Besides this daily lagging

with the moon, the highest point TIDE-WAVES BEHIND THE MOON. of the tide-wave is found to be

about 45° behind or east of the moon, so that high tide does not occur till about three hours after the moon has crossed the meridian. The waters do not at once yield to the impulse of the moon's attraction, but continue to rise after she has passed over.

B

In the cut, the moon is on the meridian, but the highest point of the wave is at A, or 450 east of the meridian; and the corresponding wave on the opposite side at B is equally behind.

284. The time and character of the tides are also affected by winds, and by the situation of different places, Strong winds may either retard or hasten the tides, or may increase or diminish their hight; and if a place is situated on a large bay, with but a narrow opening into the sea, the tide will be longer in rising, as the bay has

288. What daily lagging of the tides? Interval between two successive high tides? What other lagging? Cause of this last?

284. What modification of the time and character of the tides ?

to fill up through a narrow gate. Hence it is not usually high tide at New York till eight or nine hours after the moon has passed the meridian.

285. As both the sun and moon are concerned in the production of tides, and yet are constantly changing their positions with respect to the earth and to each other, it follows that they sometimes act against each other, and measurably neutralize each other's influence; while at other times they combine their forces, and mutually assist each other. In the latter case, an unusually high tide occurs, called the Spring Tide. This happens both at new and full moon.

CAUSE OF SPRING TIDES.

B

1. Here the sun and moon, being in conjunction, unite their forces to produce an ex. traordinary tide. The same effect follows when they are in opposition; so that we have two spring tides every month-namely, at new and full moon.

2. If the tide-waves at A and B are one-third higher at the moon's quadrature than usual, those of C and D will be one-third lower than usual.

286. Although the sun attracts the earth much more powerfully, as a whole, than the moon does, still the moon contributes more than the sun to the production of tides. Their relative influence is as one to three. The nearness of the moon makes the difference of her attraction on different sides of the earth much greater than the difference of the sun's attraction on different sides.

It must not be forgotten that the tides are the result not so much of the attraction of the sun and moon, as a whole, as of the difference in their attraction on different sides

285. Do the sun and moon always act together in attracting the waters? Why not? How affect each other's influence? Effect on the tides? What are Spring Tides? When do they occur? (Illustrate by diagram the cause of spring tide, when the sun and moon are in conjunction.)

286. Comparative influence of sun and moon in the production of tides? Why moon's influence the greatest? (Substance of note? Demonstration.)

30

of the earth, caused by a difference in the distances of the several parts. The attraction being inversely, as the square of the distance (69), the influence of the sun and moon, respectively, must be in the ratio of the earth's diameter to their distances. Now the difference in the distance of two sides of the earth from the moon isth of the moon's distance; as 240,000÷8,000 = 30; while the difference, as compared with the distance of the sun, is only 5th, as 95,000,000 ÷8,000=11,875.

287. When the moon is in quadrature, and her influence is partly neutralized by the sun, which now acts against her, the result is a very low tide, called Neap Tide.

The whole philosophy of spring and neap tides may be illustrated by the annexed diagram.

1. On the right side of the cut, the sun and moon are in conjunction, and unite to produce a spring tide.

2. At the first quarter, their attraction acts at right angles, and the sun, instead of contributing to the lunar tide-waves, detracts from it to the amount of his own attractive force. The tendency to form a tide of his own, as represented in the figure, reduces the moon's wave to the amount of one-third.

3. At the full moon, she is in opposition to the sun, and their joint attraction acting again in the same line, tends to clongate the fluid portion of the earth, and a second spring tide is produced.

4. Finally, at the third quarter, the sun and moon act against each

SPRING AND NEAP TIDES.

other again, and the second neap tide is the result. Thus we have two spring and two neap tides during every lunation—the former at the moon's syzygies, and the latter at her quadratures.

TIDES AFFECTED BY DECLINA

TION.

288. The tides are subject to another periodic variation, caused by the declination of the sun and moon north and south of the equator. As the tendency of the tide-wave is to rise directly under the sun and moon, when they are in the south, as in winter, or in the north, as in summer, every alternate tide is higher than the intermediate one.

At the time of the equinoxes, the sun being over the equator, and the moon within 50 of it, the crest of the great tide-wave will be on the equator; but as the sun and moon decline south to A, one tide-wave forms in

the south, as at B, and the opposite one in the north, as at C. If the declination was north, as shown at D, the order of the tides would be reversed. The following diagram,

287. What are Neap Tides? Their cause? (Illustrate entire philosophy by diagram.)

288. What other periodic variations mentioned? (Explain cause, and illustrate.)

if carefully studied, will more fully illustrate the subject of the alternate high and low tides, in high latitudes, in winter and summer:

1. Let the line A A represent the plane of the ecliptic, and B B the equinoctial. On the 21st of June, the day tide-wave is north, and the evening wave south, so that the tide following about three hours after the sun and moon will be higher than the intermediate one at 3 o'clock in the morning.

2. On the 23d of December, the sun and moon being over the southern tropic, the highest wave in the southern hemisphere will be about 3 o'clock P. M., and the lowest about 3 o'clock A. M.; while at the north, this order will be reversed. It is on this account that in high latitudes every alternate tide is higher than the intermediate ones; the evening tides in summer exceeding the morning tides, and the morning tides in winter exceeding those of evening.

289. All spring and neap tides are not alike as to their elevation and depression. As the distances of the sun and moon are varied, so are the tides varied, especially by the variations of the moon.

1. At A, the earth is in aphelion, and the moon in apogee. As both the sun and moon are at their greatest distances, the earth is least affected by their attraction, and the spring tides are proportionately low.

2. At B, the earth is in perihelion, and the moon in perigee; so that both the sun and moon exert their greatest influence upon our globe, and the spring tides are highest, as shown in the figure. In both cases, the sun and moon are in conjunction, but the variation in the distances of the sun and moon causes variations in the spring tides.

290. In the open ocean, especially the Pacific, the tide rises and falls but a few feet; but when pressed into narrow bays or channels, it rises much higher than under ordinary circumstances.

289 Are all spring and neap tides alike? By what are they modified? (Illustrate by diagram.)

290. Hight of tides in open seas? How in narrow bays and channels? (Hight at different points on our coast?)

The average elevation of the tide at several points on our coast is as follows:

Cumberland, head of the Bay of Fundy...

Boston..

New Haven..

New York

Charleston, S. C...

71 feet. 114"

8

66

5 66

6 66

291. As the great tide-waves proceed from east to west, they are arrested by the continents, so that the waters are permanently higher on their east than on their west sides. The Gulf of Mexico is 20 feet higher than the Pacific Ocean, on the other side of the Isthmus; and the Red Sea is 30 feet higher than the Mediterranean. Inland seas and lakes have no perceptible tides, because they are too small, compared with the whole surface of the globe, to be sensibly affected by the attraction of the sun and moon.

We have thus stated the principal facts connected with this complicated phenomenon, and the causes to which they are generally attributed. And yet it is not certain that the philosophy of tides is to this day fully understood. La Place, the great French mathematician and astronomer, pronounced it one of the most difficult problems in the whole range of celestial mechanics. It is probable that the atmosphere of our globe has its tides, as well as the waters; but we have no means, as yet, for definitely ascertaining the fact.

CHAPTER IX.

OF COMETS.

292. COMETS are a singular class of bodies, belonging to the solar system, distinguished for their long trains of light, their various shapes, and the great eccentricity of their orbits. Their name is from the Greek coma, which

291. Direction of tide-waves? What result? Instances cited? Have inland seas and lakes any tides? Why not? Remarks respecting philosophy of tides? Of La Place? Atmospheric tides?

292. What are comets? Derivation of name? Are they opake or selfluminous ?