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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 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.
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
Charleston, S. C.
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 attraction of the Sun and Moon.
612. 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 higher
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
THE SEASONS-DIFFERENT LENGTHS OF THE DAYS AND
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 ECLIPTIO.
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? Temperature of the Earth? 615. When does any place gain heat, and when lose? Upon what does
librium of temperature would not be supported. Whenever the Sun remains more than twelve hours above the horizon of any place, and less beneath, the general temperature of that place will be above the mean state; when the reverse takes place, the temperature, for the same reason, will be below the mean state. Now, the continuance of the Sun above the horizon of any place, depends entirely upon his declination, or altitude at noon.
616. About the 20th of March, when the Sun is in the vernal equinox, and consequently has no declination, he rises at six in the morning and sets at six in the evening; the day and night are then equal, and as the Sun continues as long above our horizon as below it, his influence must be nearly the same at the same latitudes, in both hemispheres.
From the 20th of March to the 21st of June, the days grow longer, and the nights shorter, in the northern hemisphere; the temperature increases, and we pass from spring to midsummer; while the reverse of this takes place in the southern hemisphere. From the 21st of June to the 23d of September, the days and nights again approach to equality, and the excess of temperature in the northern hemisphere above the mean state, grows less, as also its defect in the southern; so that, when the Sun arrives at the autumnal equinox, the mean temperature is again restored.
the length of the days depend? 616. How about the 20th of March? From March 20th to June 21st? From June 21st to September 23d? December 21st? From December 21st to March 20th?
617. From September 23d to How with the seasons in the
the dreariness of winter to the mildness of spring, when the seasons are completed, and the mean temperature is again restored. The same vicissitudes transpire, at the same time, in the southern hemisphere, but in a contrary order. Thus are produced the four seasons of the year.
In the preceding cut, the Earth is shown in her orbit, with her axis inclined 23°; the North Pole being towards the eye of the student. At A and B the Sun shines from pole to pole, and the days and nights are equal in both hemispheres. On the right, the North Pole is in the light, and we have summer in the northern hemisphere. On the left, the reverse is the case. And the gradual shortening or lengthening of the days, and the change of temperature, are produced by the passage of the Earth from one point to another, with her axis thus inclined.
618. But I have stated not the only, nor, perhaps, the most efficient cause in producing the heat of summer and the cold of winter. If, to the inhabitants of the equator, the Sun were to remain 16 hours below their horizon, and only 8 hours above it, for every day of the year, it is certain they would never experience the rigors of our winter; since it can be demonstrated, that as much heat falls upon the same area from a vertical Sun in 8 hours, as would fall from him, at an angle of 60°, in 16 hours.
Now, as the Sun's rays fall most obliquely when the days are shortest, and most directly when the days are longest, these two causes-namely, the duration and intensity of the solar heat, together, produce the temperature of the different seasons. The reason why we have not the hottest temperature when the days are longest, and the coldest temperature when the days are shortest, but in each case about a month afterwards, appears to be, that a body once heated, does not grow cold instantaneously, but gradually, and so of the contrary. Hence, as long as more heat comes from the Sun by day than is lost by night, the heat will increase, and vice versa.
BEGINNING AND LENGTH OF THE SEASONS.
h. m. 8.
Sun enters 45 (Winter begins) 1849, December 21, 7 25 46 M. T. Wash.
1850, March 20, 8 56 88
June 21, 6 8 9
22, 19 58 21
December 21, 18 21 57
southern hemisphere ? 618. Is the simple fact that a place is enlightened by the Sun, a sufficient cause for its being warm? What circumstance determines the intensity of the Sun's rays? Why, then, is it not warmest during the longest days, and on the contrary coldest during the shortest days? How long will heat increase?
Mean or average length of the tropical year, 865 5 48 48
619. The north pole of the Earth is denominated the elevated pole, because it is always about 234° above a perpendicular to the plane of the equator, and the south pole is denominated the depressed pole, because it is about the same distance below such perpendicular.
As the Sun cannot shine on more than one-half the Earth's surface at a time, it is plain, that when the Earth is moving through that portion of its orbit which lies above the Sun, the elevated pole is in the dark. This requires six months, that is, until the Earth arrives at the equinox, when the elevated pole emerges into the light, and the depressed pole is turned away from the Sun for the same period. Consequently, there are six months day and six months night, alternately, at the poles.
620. When the Sun appears to us to be in one part of the ecliptic, the Earth, as seen from the Sun, appears in the point diametrically opposite. Thus, when the Sun appears in the vernal equinox at the first point of Aries, the Earth is actually in the opposite equinox at Libra. The days and nights are then equal all over the world. (See the cut, pages 288 and 292.)
As the Sun appears to move up from the vernal equinox to the summer solstice, the Earth actually moves from the autumnal equinox down to the winter solstice. The days now lengthen in the northern hemisphere, and shorten in the southern. The Sun is now over the north pole, where it is mid-day, and opposite the south pole, where it is midnight. As the Sun descends from the summer solstice towards the autumnal equinox, the Earth ascends from the winter solstice towards the vernal equinox. The summer days in the northern hemisphere having waxed shorter and shorter, now become again of equal length in both hemispheres.
621. While the Sun apears to move from the autumnal equinox down to the winter solstice, the Earth passes up from the vernal equinox to the summer solstice; the south pole comes into the light, the winter days continually shorten in the northern hemisphere, and the summer days as regularly increase in length in the southern hemisphere.
While the Sun appears again to ascend from its winter solstice to the vernal equinox, the Earth descends from the Summer solstice to the autumnal equinox. The summer days now shorten
619. Which is the elevated pole, and why? The depressed, and why? How are the seasons produced? 620. How are the Earth and Sun situated in the ecliptic, with reference to each other? What said of the Sun's apparent motion around the zodiac? 621. What further description of the Sun's apparent progress?