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course, cannot cast any shadows. During the increase of the moon from new to full we always see the bright or illuminated part nearest to the sun, and the remarkable spots always preserve the same position; that is, if any particular spot is seen near the east or west side or limb of the moon on any particular day, the same is always seen there. From these observations we conclude, that the moon always shows to us the same side or face, and that she has no light of her own, but shines with a light borrowed from the sun.

Some astronomers have supposed the dark spots seen upon the moon's surface to be water, but later observers deny the existence upon her surface of anything like deep water, although there are large portions of her surface which are perfectly level, but they are supposed to be alluvial tracts of land.

The moon is supposed not to have any clouds, or even any indications of an atmosphere, because nothing of the kind can be seen, as it could not fail to be, if there were any. Philosophers conclude from this, that its climate must be very singular, consisting of only two variations; one very hot, and the other very cold. As the lunar year consists of only what we call a lunar month, or 29 days, they must have a fortnight of intense heat, greater than is experienced on any portion of our planet, and a fortnight of the keenest frost, exceeding even our Greenland winter. This of course must produce absolute aridity under the sun, and a constant accumulation of ice in the opposite part of the moon. It is possible, however, that there may be a narrow region between these two extremes, where there is running water. It is possible also, as evaporation produces ice or cold, and condensation heat, that some degree of equilibrium in the temperature may be kept up, and thus, in some degree, modify the otherwise severe effects of their climates.

With the most powerful telescopes no indications of inhabitants, as would be shown by edifices, or changes on the surface of the soil, can be distinguished; in fact, telescopes must be greatly improved, before we can even hope for any such discoveries, for a line only one second

in length at the moon, as seen from the earth, would be in fact at the moon about one mile in length-or to make it more palpable, a line at the moon one mile in length will appear to us to be only about the 1,300,000th part of a mile, and even the whole diameter of the moon, when nearest to us, as seen from the earth, is only 33 minutes, while its actual diameter is about 2160 miles.

The volume or bulk of the moon is about one-fortieth of that of the earth, but as there is a difference between bulk and density, the density of the moon is only about one sixty-ninth of that of the earth; therefore bodies upon her surface must have but a feeble gravitation; one pound at the earth, if carried to the moon, would weigh only one quarter of a pound, and muscular force, as existing upon our planet, would there go much further in overcoming weight, than upon the earth.

In a former tract on Gravity, the general laws of which were there explained, it was stated that the gravity of bodies above the surface of the earth, decreases as the square of the distance from the earth's centre increases; therefore, there would be a certain point between the earth and moon where a ball, if placed there, would not fall towards either body-but if moved beyond this point, the ball would fall towards that body towards which the movement was made, and calculation shows that this point would be about 632.70 miles from the earth. This fact,

in conjunction with the existence of burning mountains in the moon, has led some philosophers to conjecture that the stones called aerolites, which at times fall to the earth, came from the moon, and were projected by the volcanoes; and this conclusion is not against all reason, for computa. tion shows that if these stones have a velocity of about 8200 feet per second, it would cause them to pass from the moon to the earth.

We have already stated, above, that the moon has no clouds, and that her atmosphere must be extremely attenuated, if she has any at all-it must be more rare than we can produce with our best air-pumps. The light of the moon is only one 300,000th part of that of the sun. Thus much have we collected relating to the physical part of

the subject of our inquiries, and upon this head we shall add but a few words more relating to the appearance of our planet to the inhabitants of the moon, if there are any. To them the earth must appear of great magnitude, nearly two degrees, or about four times larger than the moon appears to us; and to them our earth will exhibit the same phases as the moon does to us, but immovably fixed in their sky, while the stars will appear to pass slowly behind and beside it. Our earth, no doubt, will appear to them as broken into irregular portions, and surrounded by belts corresponding to our trade winds. It is doubted whether the outlines of our continents and seas can ever be distinctly discerned from the moon.

MOTION O F THE MOON.

That motion of the moon which is most familiar to man, is her revolution about the earth once in about a month. The moon moves about the earth in a curved line. The time occupied in moving from any given star to the same again is 27 days, 7 hours, 43 minutes, and 11 seconds, and this is called a sidereal revolution; a comparison of this period of time with what it was in ancient times, has proved that the moon moves faster now than she did formerly. The time which the moon takes to perform a complete revolution from any particular point of her orbit to the same again, is about the same as for a sidereal revolution. The time from one new moon to another is 29 days, 12 hours, 44 minutes; this is called her synodical revolution. The mean distance of the moon from the earth is about 237,000 miles; we say mean distance, because sometimes she is nearer than at other times. The revolution of the moon about the earth, and its comparative position with respect to the sun and earth, cause what are called its phases. When the moon is in conjunction (or new moon) she is invisible, having her dark side turned entirely towards us. The different phases of the moon may be easily made visible, and well illustrated by a simple contrivance, as follows. Place a round body, as an apple or ball, upon a support, as the top of an empty

candlestick, and place this with a lighted candle upon a table, so that the round body may be a little higher than the light; then have a second ball attached to a string, or piece of wood, or wire. Now move the second ball around the first, but not parallel to the table, but a little obliquely ; and when this movable ball comes between the light and the fixed ball, it will be seen that no light will be reflected to the fixed one. In the same manner it may be shown, that when the fixed ball is between the light and movable ball, the whole surface which is towards the fixed ball will be enlightened. Now the light represents the sun, the fixed ball the earth, and the movable ball the moon; so that when the moon is in opposition (or full) she will have her enlightened disk towards the earth. The same illustration will also show, that when the moon is midway between full and new, she will only show a part of her disk; she is then said to be in her quadratures. Thus then, during an entire revolution of the moon about the earth, she presents to us all the possible differences of face, between a disk entirely enlightened and one completely obscured.

By the same illustration, we may see what the phases of the earth would be, if viewed from the moon. When the moon is full, the earth will be obscured, if viewed from the moon; and when the moon is dark or obscured, the earth would appear full. When we see the moon just after conjunction, as two days after change, an observer at the moon would see the earth almost full. The light of the moon is derived from the sun, which is reflected from her to the earth, and a small part of it is reflected by the earth back again to the moon, and thus causes the whole contour of the moon to be dimly seen, even when but a small part of her disk is really enlightened.

It is supposed by many learned men (and with much plausibility) that the regular succession of the changes of the moon first suggested the division of time called a month; in point of fact, the Greek word menos, from which our word month is said to be derived, also means the moon. Some others have suggested that the quarterly phases of the moon, happening as they do every seventh day, first gave rise to the almost universal weekly division of time, It

will be remembered that the Deity himself gave the weekly division to his chosen people, the Jews; but other people who were ignorant of the revelation to Moses, had the same period of time.

During the journey of the moon round the earth, she is sometimes nearer to the sun than at other times; and as the sun is the centre of attraction, the moon must be more attracted towards the sun when she is near, than when further off. All these influences of the sun produce different effects, according to the relative situations of these bodies. When the moon is in opposition, or full, she is further from the sun, of course not so much attracted as the earth; therefore she moves in a straighter line and not so fast as when she is in opposition or nearer to the sun. The moon being so near to the earth, the inequalities in her motion are very great when compared with the other heavenly bodies, and thus renders the calculation of her motion and place very difficult. It is impossible to explain the cause or effect of these irregularities, without the assistance of the higher mathematics. Sufficient, we trust, has been said of her motion to give a popular view of this part of our subject.

TIDES.

The subject of the tides we have before glanced at in Vol. II. No. 7, of the Tracts, but shall now attempt an explanation of them in a more detailed manner.

The most remarkable apparent effect produced upon the earth by the motion of the moon, is that which causes a periodical flux and reflux of the waters of the ocean, called tides.

The moon revolves in its orbit about the earth, and in consequence of the universal law of gravitation, attracts or draws the earth and all its parts towards herself; but as the solid parts of the earth have cohesion, one part cannot be drawn towards the moon, without drawing all the other parts; but it is different with the liquid parts-they having but little cohesion, and their parts moving freely among themselves, one part may be attracted and drawn away from those parts which are more distant from the

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