The number of asteroids discovered up to present date is about 465. A number of these small planets have not been observed since their discovery, and are practically lost. Consequently it is now sometimes a matter of doubt, until the elements have been computed, whether the supposed new planet is really new, or only an old one rediscovered.

Our Moon.

Or all the secondary planets the earth's satellite is by far the most interesting and important. The moon completes her circuit around the earth in a period whose mean or average length is 27 days 7 hours 43. 2 minutes; but in consequence of her motion in common with the earth around the sun, the mean duration of the lunar month, that is, the time from new moon to new moon, is 29 days 12 hours 44. 05 minutes, which is called the moon's synodical period. If the earth were motionless in space the moon's orbit would be nearly an ellipse, having the earth in one of the foci; hence her distance from the earth varies during the course of a lunar month. Her mean distance from the earth is 238,850 miles. Her maximum distance, however, may reach 252,830 miles, and the least distance to which she can approach the earth is 221.520 miles. Her diameter is 2, 162 miles, and if we deduct from her distance from the earth the sum of the two radii of the earth and moon, viz., 3,962 and 1,081 miles respectively, we shall have for the nearest approach of the surfaces of the two bodies 216, 477 miles. Her orbit is a very intricate one, because the earth in moving around the sun carries the moon along with it; hence the latter is sometimes within and sometimes without the earth's orbit. Its form is that of a serpentine curve, always concave toward the sun, and inclined to the plane of the earth's orbit at an angle of 50 9', in consequence of which our satellite appears sometimes above and sometimes below the plane of the earth's orbit, through which she passes twice in a revolution. These points or positions are called nodes, and no two consecutive nodes occupy positious diametrically opposite on the lunar orbit. The nodes have a retrograde motion, which causes them to make an entire revolution in 18 years 218 days 21 hours 22 minu'es and 46 seconds. This motion was well known to the ancients, who called it the Saros, and was made use of by them in roughly predicting eclipses.

The moon always presents the same face to us, as is evident from the permanency of the various markings on its surface. This circumstance proves that with respect to the earth she revolves on an axis, and the time of rotation is exactly equal to the time of revolution around the earth, viz., 27.32166 days. The moon's axis is not perpendicular to the plane of her orbit, but deviates therefrom by an angle of about 60 41'. In consequence of this fact, and of the inclination of the lunar orbit to that of the ecliptic, the poles of the moon lean alternately to and from the earth. When the north pole leans toward the earth we see somewhat more of the region surrounding it, and somewhat less when it leans the contrary way. This displacement is known by the name of libration in latitude.

The moon's motion on her axis is uniform, but her angular velocity in her orbit is subject to slight variations by reason of the form of her orbit; hence it happens that we sometimes see a little more of the eastern or western edge at one time than at another. This phenomenon is known as libration in longitude.

The moon's surface contains about 14,685,000 square miles, or nearly four times the area of Europe. Her volume is 1-49 and her mass 1-81 that of the earth, and hence her density is about 3-5 that of the earth, or about 3 2-5 that of water. At the lunar surface gravity is only 3-20 of what it is at the earth, and therefore a body which weighs 20 pounds here would weigh only 3 pounds there. The centre of gravity of the earth and moon, or the point about which they both actually revolve in their course around the sun, lies within the earth; it is 1,063 miles below the surface.

The attractive force of the moon acting on the water of our oceans is mainly instrumental in raising them into protuberances or tides in such a manner as to give the total mass a spheroidal figure whose principal axis would continually coincide with the line joining the centres of the earth and moon, but in consequence of the resistance which this movement of the water encounters from continents and islands, as well as from the liquid molecules themselves, the tidal wave can never arrive at any place until about one hour after the moon has crossed the meridian of the place.

The moon has no atmosphere and no water. The suddenness with which stars are occulted by the moon is regarded as a conclusive proof that a lunar atmosphere does not exist, and the spectroscope furnishes negative evidence of the same character.

In remote ages the lunar surface was the theatre of violent volcanic action, being elevated into cones and ridges exceeding 20,000 feet high, and at other places rent into furrows or depressions of corresponding depth. The lunar volcanoes are now extinct. A profound silence reigns over the desolate and rugged surface. It is a dead world, utterly unfit to support animal or vegetable life.

THE EARTH'S ATMOSPHERE.

THE earth's sensible atmosphere is generally supposed to extend some forty miles in height, probably farther, but becoming at only a few miles from the surface of too great a tenuity to support life. The condition and motions of this aerial ocean play a most important part in the determination of climate, modifying, by absorbing, the otherwise intense heat of the sun, and, when laden with clouds, hindering the earth from radiating its acquired heat into space. - Whitaker.