spins rapidly, its uppermost part appears as a point. As it falls, the point becomes a small circle, increasing as the rotation slackens. But if the coin be very closely watched, when beginning to fall, it will be seen that the small circle is for a moment diminished, showing that the coin had partially recovered its upright position. This recovery is entirely due to the rotation. Similarly, a bicycle is kept from falling by its horizontal motion; and a conical bullet, which has gained a great rapidity of rotation from a rifled barrel, keeps the direction of its axis without deflection to the right or left. And thus we find that the present position of the earth’s axis with respect to the ecliptic is not altered; but the two forces acting upon the earth cause the axis to rotate, as above described, so that the north pole describes a circle in the heavens. But as the period of this rotation is very great, it was not easy to detect such a result, except after a long period of observation. It was discovered thus. The point where the ecliptic and equator cut is called the first point of the constellation Aries, one of the well-known twelve signs of the zodiac. From this point all celestial measurements are made eastwards. Each star of importance has had its distance east of that point—called its right ascension—recorded. In the course of time, the tables of these numbers so recorded appeared to be erroneous; but the error was so regular, and all in one direction, that it was conjectured that the point from which these right ascensions were reckoned had itself shifted its place. And so it proved; and if any one looks at a celestial globe, he will see that Aries no longer occupies the position where the equinox is, but is somewhat to the east, or right, because the point of intersection of the ecliptic and equator has slipped back. But as the sun appears to take a shorter time to come back to the equinox than to arrive at the same stars, which were once close to that point of intersection, this slow retrograde motion is termed the _precession_ of the equinoxes. The distance on the equator caused by this retrograde motion would, if not otherwise modified, be 50″·41 annually. But the attraction of the planets on each other produces a very small motion of the equinox in the other direction; and so the resulting precession is about 50″·1 annually. If we divide the three hundred and sixty degrees in every circle by the above small quantity, we shall find that the period of the revolution of the earth’s axis is twenty-five thousand eight hundred and sixty-eight years.