that hemisphere of the earth which is next the moon, and that the moon's disc will be equally obscured, from whatever point

It

it is seen. When the moon passes through only a part of the earth's shadow, then she suffers only a partial eclipse, but this is also visible from the whole hemisphere next the moon. will be remembered that lunar eclipses happen only at full moon, the sun and moon being in opposition, and the earth between them.

Solar Eclipses.-When the moon passes between the earth and sun, there happens an eclipse of the sun, because then the moon's shadow falls upon the earth. A total eclipse of the sun happens often, but when it occurs, the total obscurity is confined to a small part of the earth: since the dark portion of the moon's shadow never exceeds 200 miles in diameter on the earth. But the moon's partial shadow, or penumbra, may cover a space on the earth of more than 4000 miles in diameter, within all which space the sun will be more or less eclipsed. When the penumbra first touches the earth, the eclipse begins at that place, and ends when the penumbra leaves it. But the eclipse will be total only where the dark shadow of the moon touches the earth.

Earth

Fig. 207.

Eclipse of the Sun

Sun

Why is the same eclipse total at one place, and only partial at another? Why is a total eclipse of the sun confined to so small a part of the earth?

Fig. 207 represents an eclipse of the sun, without regard to the penumbra, that it may be observed how small a part of the earth the dark shadow of the moon covers. To those who live within the limits of this shadow, the eclipse will be total, while to those who live in any direction around it, and within reach of the penumbra, it will be only partial.

Solar eclipses are called annular, from annulus, a ring, when the moon passes across the centre of the sun, hiding all his light, with the exception of a ring on his outer edge, which the moon is too small to cover from the position in which it is seen. Fig. 208.

Fig. 208 represents a solar eclipse, with the penumbra D, C, and the umbra, or dark shadow, as seen in the above figure. When the moon is at its greatest distance from the earth, its shadow mo, sometimes terminates, before it reaches the earth, and then an observer standing directly under the point o, will see the outer edge of the sun, forming a bright ring around the circumference of the moon, thus forming an annular eclipse.

The penumbra D C, is only a partial interception of the sun's rays, and in annular eclipses it is this partial shadow only which reaches the earth, while the umbra, or dark shadow, terminates in the air. Hence annular eclipses are never total in any part of the earth. The penumbra, as already stated, may cover more than 4000 miles of space, while the umbra never covers more than 200 miles in diameter; hence partial eclipses of the sun may be seen by a vast number of inhabitants, while comparatively few will witness the total eclipse.

When there happens a total solar eclipse to us, we are eclipsed to the moon, and when the moon is eclipsed to us, an eclipse of the sun happens to the moon. To the moon, an eclipse

What is meant by penumbra? What will be the difference in the aspect of the eclipse, whether the observer stands within the dark shadow, or only within the penumbra? What is meant by annular eclipses? Are annular eclipses ever total in any part of the earth? In annular eclipses, what part of the moon's shadow reaches the earth?

of the earth can never be total, since her shadow covers only a small portion of the earth's surface. Such an eclipse, therefore, at the moon, appears only as a dark spot on the face of the earth; but when the moon is eclipsed to us, the sun is partially eclipsed to the moon for several hours longer than the moon is eclipsed to us.

The Tides.

The ebbing and flowing of the sea, which regularly takes place twice in 24 hours, are called the tides. The cause of the tides, is the attraction of the sun and moon, but chiefly of the moon, on the waters of the ocean. In virtue of the universal principle of gravitation, heretofore explained, the moon, by her attraction, draws, or raises the water towards her, but because the power of attraction diminishes as the squares of the distances increase, the waters, on the opposite side of the earth, are not so much attracted as they are on the side nearest the moon. This want of attraction, together with the greater centrifugal force of the earth on its opposite side, produced in consequence of its greater distance from the common centre of gravity, between the earth and moon, causes the waters to rise on the opposite side, at the same time that they are raised by direct attraction on the side nearest the moon.

Thus the waters are constantly elevated on the sides of the earth opposite to each other above their common level, and consequently depressed at opposite points equally distant from these elevations.

Let m, fig. 209, be the moon, and E the earth covered with Fig. 209.

S

m

d

water. As the moon passes round the earth, its solid and fluid parts are equally attracted by her influence according to their densities; but while the solid parts are at liberty to move only as a whole, the water obeys the slightest impulse, and thus tends towards the moon where her attraction is the strongest.

What is said concerning eclipses of the earth, as seen from the moon What are the tides? What is the cause of the tides? What causes the tide to rise on the side of the earth opposite to the moon?

Consequently the waters are perpetually elevated immediately under the moon. If therefore the earth stood still, the influence of the moon's attraction would raise the tides only as she passed round the earth. But as the earth turns on her axis every 24 hours, and as the waters nearest the moon, as at a, are constantly elevated, they will, in the course of 24 hours, move round the whole earth, and consequently from this cause there will be high water at every place once in 24 hours. As the elevation of the waters under the moon causes their depression at 90 degrees distance on the opposite sides of the earth d and c, the point c will come to the same place, by the earth's diurnal revolution, six hours after the point a, because c is one quarter the circumference of the earth from the point a, and therefore there will be low water at any given place six hours after it was high water at that place. But while it is high water under the moon, in consequence of her direct attraction, it is also high water on the opposite side of the earth in consequence of her diminished attraction, and the earth's centrifugal motion, and therefore it will be high water from this cause twelve hours after it was high water from the former cause, and six hours after it was low water from both causes.

Thus, when it is high water at a and b, it is low water at c and d, and as the earth revolves once in 24 hours, there will be an alternate ebbing and flowing of the tide, at every place, once in six hours.

But while the earth turns on her axis, the moon advances in her orbit, and consequently any given point on the earth will not come under the moon on one day so soon as it did on the day before. For this reason, high or low water at any place comes about fifty minutes later on one day than it did the dav before.

Thus far we have considered no other attractive influence except that of the moon, as affecting the waters of the ocean. But the sun, as already observed, has an effect upon the tides, though on account of his great distance, his influence is small when compared with that of the moon.

When the sun and moon are in conjunction, as represented in fig. 209, which takes place at her change, or when they are

If the earth stood still, the tides would rise only as the moon passes round the earth; what, then, causes the tides to rise twice in 24 hours? When it is high water under the moon by her attraction, what is the cause of high water on the opposite side of the earth, at the same time? Why are the tides about 50 minutes later every day?

in opposition, which takes place at full moon, then their forces are united, or act on the waters in the same direction, and consequently the tides are elevated higher than usual, and on this account are called spring tides.

But when the moon is in her quadratures, or quarters, the attraction of the sun tends to counteract that of the moon, and although his attraction does not elevate the waters and produce tides, his influence diminishes that of the moon, and consequently the elevation of the waters are less when the sun and moon are so situated in respect to each other, than when they are in conjunction, or opposition.

This effect is represented by fig. 210, where the elevation of the tides at c and d is produced by the causes already explained; but their elevation is not so great as in fig. 206, since the influence of the sun acting in the direction a b, tends to counteract the moon's attractive influence. These small tides are called neap tides, and happen only when the moon is in her quadratures.

The tides are not at their greatest heights at the time when the moon is at its meridian, but sometime afterwards, because the water, having a motion forward, continues to advance by its own inertia, sometime after the direct influence of the moon has ceased to affect it.

Latitude and Longitude.

Latitude is the distance from the equator in a direct line, north or south, measured in degrees and minutes. The number of degrees is 90 north, and as many south, each line on which these degrees are reckoned running from the equator to the poles. Places at the north of the equator are in north latitude, and those south of the equator are in south latitude. The parallels of latitude are imaginary lines drawn parallel to What produces spring tides? Where must the moon be in respect to the sun, to produce spring tides? What is the occasion of neap tides? What is latitude?