then when it was perfectly still, the thread was burned off,

and the ball began to oscillate. through the ridges of sand, it

Fig. 8.

As the pointer passed was seen that at each

vibration it crossed a little to the right, looking from the center, of the place where it crossed before.

26. Theory of Foucault's pendulum.— When the pendulum begins to swing, being drawn only by the earth's attraction, it must move in the plane which contains the three points, the point of suspension, the point from which it started, and the center of the earth. It can not of

end of the vibration. The next vibration begins in the same plane, and, therefore, like the first, ends in it, and so each subsequent vibration. That to which the pendulum

is suspended has, of course, the motion of the earth at the place where the experiment is performed; but, as the wire is so fastened that no rotary or twisting motion can be communicated to it, the forward motion of the point of suspension only carries the plane of vibration forward, without twisting it to the right or left.

The ball, then, does not move toward the right; its apparent motion makes visible the actual motion of the earth beneath it, toward the left, that is, toward the east.

Fig. 9.

Fig. 10.

27. The experiment at different places on the earth.

At the pole, the point of suspension does not move; the plane of vibration is fixed, and the earth rotates beneath.

At the equator, the meridians being perpendicular to the equator are parallel to each other, and hence have always the same position relative to the plane of vibration.

Between the equator and the pole, the meridians converge, or, what amounts to the same, the new positions which each meridian takes as the earth rotates, make with the old, angles

which constantly increase. Hence, as the pendulum maintains its position, although it may have been started

Fig. 11.

in the plane of one of these meridians, an angle is soon formed between them which constantly increases until they again coincide

after 24 hours.

This experiment was performed at Paris in the Pantheon, and in the Bunker Hill Monument near Boston.

28. The Gyroscope when rotating in a ver

tical plane remains always in that plane. M. Foucault used this instrument, watching its turning with a telescope, with the same results, explained by the same theory.

CHAPTER III.

ASTRONOMICAL IDEAS

DERIVED FROM THE MOTION OF THE EARTH.

30. Axis of the heavens.-The axis of the earth, extended in each direction until it meets the sky, becomes the axis of the heavens, or the line about which the sky seems to revolve. The points where this line meets the sky are the north and south poles of the heavens.

31. Equinoctial.-If the plane of the equator is extended every way, the line in which it meets the sky is a great circle, called the celestial equator, or EQUINOCTIAL. The stars appear to describe circles about the poles of the heavens, parallel to the equinoctial; the circles may be called circles of daily motion.

32. Meridians are great circles on the sky, perpendicular to the equinoctial, and passing through its poles. Celestial meridians must be imagined on the sky, as terrestrial meridians are imagined on the surface of the earth. As every place on the earth has its own meridian passing through the north and south poles, so each star in the heavens has its meridian passing through the poles of the heavens. When we speak of THE MERIDIAN, as when we say the sun, or a star, comes to, or passes the meridian, we refer to the plane of the terrestrial meridian of the place where the observation is made.

CO-ORDINATES.

33. Declination.-The distance of a heavenly body from the equinoctial, measured on a meridian, is called its declination. Declination, therefore, corresponds to terrestrial latitude; it is north or south declination as the

object is north or south of the equinoctial. The declination of the poles is 90°.

34. Polar distance. The polar distance of a star is its distance from the nearest pole, measured on a meridian. The declination and polar distance of the same star are together always equal to 90°.

35. Right ascension. The measurement which corresponds to terrestrial longitude is called right ascension.