destitute of support from observations until Vogel made his wonderful discovery.
Applying the improved spectrographic process to Algol, he determined on one night that Algol was retreating from the earth at a speed of twenty-six miles a second. This in itself is a striking fact, but of course the velocity is not an exceptionally large one for celestial movements. We know of one star at least which moves half a dozen times as fast. When, however, Vogel came to repeat his observations he found that Algol was again moving with the same velocity, but this time the movement was towards the earth instead of from it. Here was indeed a singular circumstance demanding the careful examination which it speedily received. It appeared that the movements of Algol to and fro were strictly periodic, that is to say, for one day and ten hours the star is moving towards us, and then for a like time it moves from us, the maximum speed in each advance or retreat being that which we have mentioned, namely twenty-six miles a second. The interest awakened by this discovery culminates when it appears that this movement to and fro is directly associated in a remarkable manner with the variation of Algol's lustre. It is invariably found that every time the movement of retreat is concluded, the star loses its brilliance, and regains it again at the commencement of the return movement. It is thus plain that the changes in brilliance of the star bear an important relation to the periodic movement. Here was an important step taken.
For the next advance in this remarkable investigation we have to depend, not on our instruments, but on the laws of mechanics. We have spoken of Algol as moving to and fro, but it is necessary to observe
