forms its revolution round the earth. The light of the sun illuminates one half of its surface, and leaves the other in darkness. Of this illumination we perceive different degrees, according to the various positions of the moon, with respect to the sun and the earth. We see one half of its body enlightened, or a full moon, when it is placed in opposition to the sun, or when the sun is in one part of the heavens, as west, and the moon in the opposite part, as east. When the moon is in conjunction with the sun, or in that part of its orbit which is between the earth and the sun, its enlightened surface is turned from us, which renders it invisible; this is the time of the new moon. When the moon appears in the intermediate part of its orbit, between the conjunction and opposition, it is in its quadratures, and about half of its illuminated surface is turned towards us.

As the moon illuminates the earth by light reflected from the sun, so she is reciprocally illuminated by the earth which reflects the sun's rays to the surface of the moon. As the surface of the earth is more than thirteen times greater than that of the moon, the earth must appear to the inhabitants of the moon thirteen times larger than the moon does to us, and it will exhibit the same phases, but in an opposite order, As the rotation of the moon on her axis is performed in the same time that she goes once round the earth,—which is evident from her always presenting the same face to us during the whole of her monthly revolution,-it is plain, that the inhabitants of one half of the lunar world are totally deprived of a sight of the earth, unless they travel to the opposite hemisphere,

The face of the moon appears to have shades of different colours. If viewed through an ordinary telescope, her surface will appear diversified with long tracts of mountains and cavities. It has been ascertained that these are mountains from the shadows which they cast, and some of them are supposed to be volcanic,

The difference between the rising of the moon on one day and the preceding is generally about fifty minutes. But in places of considerable latitude, there is a remarkable difference about the time of harvest, when at the season of full moon she rises for several nights together only about twenty minutes later on the one day than on that immediately pre ceding. By thus succeeding the sun before he twilight is

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THE HARVEST MOON.

ended, the moon prolongs the light, to the great benefit of those who are engaged in gathering in the fruits of the earth; and hence the full moon at this season is called the harvest moon. It is believed that this was observed by persons engaged in agriculture, at a much earlier period than it was noticed by astronomers. The phenomenon may be easily explained by the assistance of a globe; and it is occasioned by the moon's orbit lying sometimes more oblique to the horizon than at others.

The Harvest Moon.

All hail! thou lovely queen of night,
Bright empress of the starry sky!
The meekness of thy silvery light
Beams gladness on the gazer's eye,
While from thy peerless throne on high
Thou shinest bright as cloudless noon,
And bidd'st the shades of darkness fly
Before thy glory-Harvest moon!
In the deep stillness of the night,
When weary labour is at rest,

How lovely is the scene!-how bright

The wood-the lawn-the mountain's breast,

When thou fair moon of Harvest! hast

Thy radiant glory all unfurled,

And sweetly smilest in the west,

Far down upon the silent world.

Shine on, fair orb of light! and smile
Till autumn months have passed away,

And labour hath forgot the toil

He bore in summer's sultry ray;

And when the reapers end the day,

Tired with the burning heat of noon,
They'll come with spirits light and gay,

And bless thee-lovely Harvest Moon!
W. MILLAR.

QUESTIONS.-1. In what time does the moon revolve round the earth? 2. At what distance is it from the earth? 3. In what time does it turn on its axis? 4. What is said of the illumination of the moon? 5. How does the earth appear as seen from the moon? 6. How does the face of the moon appear when viewed through a telescope? 7. What is the Harvest Moon? 8. By what is it occasioned?-9. Look at fig. 41, and illustrate the phases of the moon.

THE sea is observed to flow for certain hours from the south towards the north. In this motion, which lasts about six hours, the sea gradually swells; so that entering the mouths of rivers, it drives back the waters towards their heads. After a continual flow of six hours, the sea seems to rest for about a quarter of an hour; it then begins to ebb, or retire back again from north to south for six hours more; and the rivers resume their natural course. Then, after a seeming pause of a quarter of an hour, the sea again begins to flow, as before, and thus alternately. This regular and alternate motion of the sea constitutes the tides. They are chiefly occasioned by the attraction of the moon, but are affected by that of the sun. There are two tides in about twenty-five hours; and the time of high or low water is every day fifty minutes later than on the preceding day. The moon is supposed to draw the earth towards itself, and to act upon the solid parts of it, in the same manner as if its whole weight were in a single point in or near the centre. Now the waters at any place over which the moon is passing, will be more attracted than the earth; and therefore will be heaped up under the moon. But the waters on the opposite side of the globe will be less attracted than the earth; consequently the earth is drawn away from them; and they are heaped up, or, in other words, it is high water there. When the waters are elevated at the side of the earth under the moon, and at the opposite side also, it is evident they must recede from the intermediate points, and thus the attraction of the moon will produce high water at two places and low water at two places on the earth at the same time. The tide is fifty minutes later every day, because it is twenty-four hours and fifty minutes before the same meridian on our globe returns beneath the moon. The earth revolves on its axis in about twenty-four hours; if the moon, therefore, were stationary, the same part of our globe would return beneath it, every twenty-four hours; but as during our daily revolution the moon advances in her orbit, the earth must make more than a complete revolution in order to bring the

same meridian opposite the moon; we are fifty minutes in overtaking her, and the tides are retarded for the same reason that the moon rises later on one day than on the preceding.

The tides, though constant, are not equal; but are greatest when the moon is in conjunction with the sun or in opposition to it, or at the time of new and full moon; and least, when in quadrature to it. This increase and diminution constitute the spring and neap tides. The attraction of the sun does not raise tides; its only effect is to increase or diminish those of the moon. The tides are highest when both the luminaries are in the equator, and the moon at the least distance from the earth. This happens at the time of the equinoxes. The tide is at the greatest height, not when the moon is in the meridian, but some time afterwards, because the force by which the moon raises the tide continues to act after it has passed the meridian. The regular tides are greatly affected by strong winds. Continents also stop them in their course from east to west, and in narrow rivers they are frequently very high and sudden, from the resistance of the banks. The advantages arising from tides are great. By agitating the waters of the ocean they preserve them in a state of purity. Aided by their means, ships of the largest burden sail up rivers against their natural course, and convey into the interior of countries those productions which stimulate the industry and promote the happiness of nations.

QUESTIONS.-1. What are the tides? 2. How are they occasioned? 3. How does it appear that the moon produces high water in two places at the same time? 4. How do you account for the tide being fifty minutes later every day? 5. What are spring and neap tides? 6. What is the effect of the sun's attraction? 7. When are tides highest? 8. What produces irregularity in tides? 9. What advantages arise from tides? 10. Look at figures 42 and 43, and explain the tides.

LESSON 49.

Eclipses.

An'nular, having the form of a ring, from annulus, a Latin word for ring.

THE earth being an opaque body enlightened by the sun, necessarily projects a shadow into the regions of space in a

ECLIPSES OF THE SUN.

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contrary direction. When it so happens that the moon, in the course of her revolution about the earth, falls into this shadow, she loses the sun's light, and appears to us eclipsed. If we suppose two straight lines drawn from the opposite parts of the solar disk, touching the surface of the earth on opposite sides; these lines will represent the limits of the shadow, and as the sun is much larger than the earth, they will meet at a point and cross each other behind the earth, and the shadow will thus take the form of a cone. The moon can come within the shadow of the earth only when it is full, or in opposition to the sun. But the moon is not eclipsed every time it is full, because its orbit does not coincide with the plane of the earth's orbit, one half being about five degrees and a third above it, and the other half as much below it; and unless the full moon, therefore, happen in or near one of the nodes, that is, in or near the points in which the two orbits intersect each other, she will pass above or below the shadow of the earth, in which case there can be no eclipse. If the moon be within twelve degrees from the node, at the time when she is full, there will be a partial or total eclipse, according as a part, or the whole of her disk falls within the earth's shadow. As the shadow is considerably wider than the moon's diameter, an eclipse of the moon lasts sometimes three or four hours. It is by knowing exactly at what distance the moon is from the earth, and of course the width of the earth's shadow at that distance, that eclipses are calculated with the greatest accuracy, for many years before they happen. Lunar eclipses are visible over every part of the earth that has the moon at that time above the horizon; and the eclipse appears of the same magnitude to all from the beginning to the end. That faint reddish colour, which the moon exhibits in the midst of an eclipse, is supposed to proceed from the rays of light, which are refracted by the earth's atmosphere, and fall upon the surface of the moon.

An eclipse of the sun is caused by an interposition of the moon between the sun and the earth. This can happen only at the new moon, or when the moon at her conjunction is near one of her nodes; for unless the moon is in or near one of her nodes, she cannot appear in the same plane with the sun, or seem to pass over his disk. In every other part of her orbit she will appear above or below the sun. If the