Page:Popular Astronomy - Airy - 1881.djvu/84

70 it was not received by those who were able to reason on the matter in more distant times.

But when the telescope was invented, fresh objects presented themselves for contemplation, and new arguments were furnished. We then obtained a sight of the planets, particularly of Jupiter. We saw that he is a spherical, or rather spheroidal planet, like the earth, but probably much bigger. We can see spots upon Jupiter, and by these we can ascertain, that he revolves in a much shorter time than the earth, or in about ten hours. Now, the knowledge of these things in later days has become a very strong argument indeed. Jupiter is a large planet that turns on his axis, and why do not we turn? We are very much alike in our general character.

But, finally, we come to another observation, founded on our determinations of the figure of the earth. We have found, from measures, that the earth is flattened at the Poles, or turnip-shaped. This leads, then, to the question—is that connected with the rotation of the earth? Most certainly it is. If we take anything circular which admits of a change of form; if, for instance, we mount a hoop, as in

Fig. 23, in such a manner that we can make it revolve