[18] Hersch. 825. In Humboldt's _Cosmos_, III. 243, Argelander, who has most carefully observed and studied these periodical stars, has given a catalogue containing 24, with the most recent determinations of their periods. [19] Hersch. 821. Humboldt (_Cosmos_, III. 238 and 246,) gives the period as 68 hours 49 minutes, and says that it is 7 or 8 hours in its less bright state. If we could suppose the times of the warning, and of the greatest eclipse, given by Herschel, to be exactly determined, as 3-1/2 and 1/4, that is, in the proportion of 14 to 1, the darkening body must have its effective breadth 14/15 of that of the star. But this is on the supposition that the orbit of the darkening body has the spectator's eye in its plane; if this be not so, the darkening body may be much larger. [20] Hersch. _Outl. Astr._ 821. Another explanation of the variable period of Algol, is that the star is moving towards us, and therefore the light occupies less and less time to reach us. [21] Humboldt, very justly, regards the force of analogy as tending in the opposite direction. "After all," he asks, (_Cosmos_, III. 373,) "is the assumption of satellites to the Fixed Stars so absolutely necessary? If we were to begin from the outer planets, Jupiter, &c., analogy might seem to require that all planets have satellites. But yet this is not true for Mars, Venus, Mercury." To which we may further add the _twenty-three_ Planetoids. In this case there is a much greater number of bodies which have not satellites, than which have them. [22] _Consolations in Travel_. Dial. 1. [23] What is said in Art. 15, that in consequence of the time employed in the transmission of visual impressions, our seeing a star is evidence, not that it exists now, but that it existed, it may be, many thousands of years ago; may seem, to some readers, to throw doubts upon reasonings which we have employed. It may be said that a star which was a mere chaos, when the light, by which we see it, set out from it, may, in the thousands of years which have since elapsed, have grown into an orderly world. To which bare possibility, we may oppose another supposition at least equally possible:--that the distant stars were sparks or fragments struck off in the formation of the Solar System, which are really long since extinct; and survive in appearance, only by the light which they at first emitted. CHAPTER IX. THE PLANETS.