Sun and on her axis. The small circles show her path around the center of gravity, and the arrows her direction.

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.

TIDE-WAVES BEHIND THE MOON.

603. 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 of the tide-wave is found to be about 46° 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.

In the cut, the Moon is on the meridian, but the highest point of the wave is at A, or 45° east of the meridian; and the corresponding wave on the opposite side at B is equally behind.

604. 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 height; 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 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.

605. 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.

608. What daily lagging of the tides? Interval between two successive high tides? What other lagging? Cause of this last? 604. What modification of the time and character of the tides? 605. 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 ure Spring Tides? When do they occur? Illustrate by diagram the cause of spring tides, when the Sun and Moon are in conjunction.

Here the Sun and Moon, being in conjunction, unite their forces to produce an extraordinary 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.

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.

606. 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.

On the right side of the cut, the Sun and Moon are in conjunction, and unite to produce a spring tide.

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.

At the full Moon, she is in opposition to the Sun, and their joint attraction acting again in the same line, tends to elongate the fluid portion of the Earth, and a second spring tide is produced.

Finally, at the third quarter, the Suu and Moon act against each 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 cyzygies, and the latter at her quadratures.

FIRST

NEAP

THIRD"

QUARTER

DEO

TIDE

QUARTE

NEAR TIDE

607. Although the Sun attracts the Earth much more power fully, 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

606. What are Neap Tides? Their cause? Illustrate entire philosophy by diagram. 607. Comparative influence of Sun and Moon in the production of tides? Why Moon' influence the greatest? Substance of ne te ? Demonstration?

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 of the Earth, caused by a difference in the distances of the several parts. The attraction being inversely as the square of the distance (558), 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 is th of the ३० Moon's distance; as 240,000+8,000=80; while the difference, as compared with the distance of the Sun, is only 75th, as 95,000,000+8,000=11,875.

TIDES AFFECTED BY DECLINA-
TION.

608. 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 5° 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, if carefully studied, will more fully illustrate the subject of the alternate high and low tides, in high latitudes, in winter and summer:

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.

On the 23d of December, the Sun and Moon being over the southern tropic, the highest wave in the southern hemisphere will be about 2 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 win. ter exceeding those of evening.

609. All spring and neap tides are not alike as to their eleva tion and depression. As the distances of the Sun and Moon are

608. What other periodic variations mentioned? Explain cause, and illustrate. 669. Are all spring and neap tides alike? By what are they modified? Illustrate Ly diagram.

varied, so are the tides varied, especially by the variations of the Moon.

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.

A. 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 thown 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.

610. 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 cir cumstances.

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. 111/4

60

611. 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 attrac tion of the Sun and Moon.

ATMOSPHERICAL TIDES.

512. Air being lighter than water, and the surface of the atmosphere being nearer to the Moon than the surface of the sea, it cannot be doubted but that the Moon raises much higner

610. Height of tides in open seas? How in narrow bays and ferent points on our coast ? 611. Direction of tide-waves? cited? Have inland seas and lakes any tides? Why not? losophy of tides? 612 Atmospheric tides?

channels? Height at difWhat result? Instances Remarks respecting phi

tides in the atmosphere than in the sea. According to Sir John Herschel these tides are, by very delicate observations, rendered not only sensible, but measurable.

Upon the supposition that there is water on the surface of the Moon of the same specific gravity as our own, we might easily determine the height to which the Earth would raise a lunar tide, by the known principle, that the attraction of one of these bodies on the other's surface is directly as its quantity of matter, and inversely as its diameter. By making the calculation, we shall find the attractive power of the Earth upon the Moon to be 21,777 times greater than that of the Moon upon the Earth.

613. 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 XIV.

THE SEASONS-DIFFERENT LENGTHS OF THE DAYS AND

NIGHTS.

614. THE vicissitudes of the seasons, and the unequal lengths of the days and nights, are occasioned by the annual revolution of the Earth around the Sun, with its axis inclined to the plane of its orbit. The temperature of any part of the Earth's surface depends mainly, if not entirely, upon its exposure to the Sun's rays.

INCLINATION OF THE EARTH'S AXIS TO THE PLANE OF THE ECLIPTIC.

615. Whenever the Sun is above the horizon of any place, that place is receiving heat; when the Sun is below the horizon it is parting with it, by a process which is called radiation. The quantities of heat thus received and imparted in the course of the year, must balance each other at every place, or the equi

618. Is it certain that this subject is even yet well understood? Remark of Laplace? 614. Cause of the seasons, and the unequal length of the days and nights? Temperatur of the Earth? 615. When does any place guin heat, and when lose? Upon what does