Page:Popular Astronomy - Airy - 1881.djvu/53

Rh the direction of the axis round which the stars appear to turn; and that we find, by fixing the telescope to the axis in such a position that it is directed to any one star, and then, by continuing to turn the instrument upon its axis, the telescope will follow the star from its rising to its setting. This I mentioned as establishing an important point, that the stars undoubtedly do appear to revolve round that axis. I then described the use of the clock-work for causing the Equatoreal instrument to revolve uniformly. And I pointed out to you, as a thing of importance, that when the clock-work is in action to whatever star we may direct the telescope, however far that star may be from the Pole, or however near it may be to the Pole, the telescope does continue to revolve after it, so that the star is always kept in sight, or in the field of view. Inasmuch, therefore, as all the stars appear to revolve uniformly round one axis, it follows that the stars keep their relative places or positions, that is to say, the heavens turn as it were all of a piece. Of course there is no explanation of that, except one of these two—either that the heavens are solid and go all of a piece; or that the heavens may be assumed to be fixed or immovable, and that we and the earth are turning instead of them.

I then particularly mentioned that, taking advantage of this circumstance, instruments are contrived for daily use in every Observatory in the world, as adapted to defining the places of celestial objects. In the first place I directed your attention to the transit instrument, as one of the most important instruments used in taking our observations. This is mounted like a cannon, turning upon two pivots, and possessing no other motion; these pivots resting