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MOUNTING OF THE TELESCOPE
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at a star and made to follow it in its diurnal motion. In order not to distract the attention of the reader by beginning a study of the instrument with a view of all the details, we first give an outline, showing the relation of the axes on which the telescope turns. The principal axis, called the polar axis, is adjusted so as to be parallel to the axis of the earth, and therefore to point at the celestial pole. Then, as the earth turns from west toward east, a clockwork connected with this axis turns the

Fig. 11.—Axes on which a Telescope turns.

instrument from east toward west, with an equal motion. Thus the rotation of the earth is neutralized, as it were, by the corresponding rotation of the telescope in the opposite direction. When the instrument is pointed at a star and the clockwork set going, the star when once found will remain in the field of view.

In order that a telescope may be directed at any point of the heavens at pleasure, there must be another axis, at right angles to the polar axis. This is called the

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