Page:Popular Astronomy - Airy - 1881.djvu/26

12 in Greenwich, Berlin, Paris, and all others of any importance, this apparatus is used; in fact, it is used also in all the leading private Observatories, and the clock work which I now exhibit is borrowed from a private Observatory—from the Observatory of Dr. Lee. A spindle KL from this apparatus is attached to the worm which carries the Equatoreal. It makes the telescope of the Equatoreal revolve round the axis uniformly, and it thus gives us the means of ascertaining, with the utmost exactness, whether it be true, or whether it be not true, that all the stars do move with equal angular speed around one imaginary axis. When this machinery is in play, the telescope is adjusted, and pointed to the star. Whether it be turned to a star near the Pole, or to a star at a distance from the Pole, the effect is this—that the star is constantly seen in the field of view of the telescope—the telescope turns just as fast as the star moves.

Observe now the results obtained from these things. The first thing I mentioned was, if the telescope be directed to a star, and the instrument be turned it follows the star in the whole of its course. The next result, which is particularly connected with the use of this machine, is, that the same uniform motion round the axis follows any of the stars, wherever you select them. This is the same as saying that the stars move, as it were, all in a piece; and when you come to examine how it bears on Astronomy, you cannot attach too great an importance to these results. It is indeed the great and fundamental principle of Astronomy, that the stars do move as if they were attached to a shell, or in other words, that they move all in a piece. As to the explanation of that, I shall not trouble you at present. I simply call attention