Yet, as in this unexplored, and, perhaps, unex- a plorable abyfs of space, it is no neceffary condition that the planets should be of the fame magnitudes as thofe belonging to our system, it is not improbable but that planetary bodies be discovered among the double and triple stars. may Our curiofity is much interested in the contem- B plation of the phenomena of new and changeable stars, but the causes that may be offered with plaufibility to folve these appearances are not many. If the light of the Sun and fixed ftars be ima- c gined to proceed from a combustion fimilar to that which is required to produce light in our experiments, it may happen that when all the inflammable matter is decomposed the ignition may cease. Or, if a mass of matter adapted for inflammation begin by any cause to burn, its ignition and emiffion of light will then commence. These considerations may explain the disappearance of some stars, and the appearance of others. And as there are no data to fix the time between the beginning and end of the appearance, the stars may last for any given time, according to circumstances. The spots on the Sun (174, v) have afforded D a conjecture respecting the cause of the periodical change of brightness in some stars. For, if a ftar be supposed to have a spot of confiderable magnitude, and to turn on its axis, it will be much brighter when the spot is not on the visible disc than when it is. However, it must be confeffed, that the phenomena do not in general agree with this fuppofition, → which cannot eafily be reconciled to the permanent brightness or obscurity that prevails in the changeable stars for more than half the period. E If a ftar, by a swift rotation, be made to affume and preserve a flattened figure, and its axis have a rotation fimilar to that hereafter to be explained in the Earth, it will be much lefs bright when its edge is presented to the observer than when the visible disc is projected broader. F Or, laftly, if a large planet revolve about a ftar, it may occafion certain periodical eclipfes of fufficient magnitude and duration to be perceived by us, on account of the quantity of light interGcepted. Thus, for example, if an opaque planet, whose diameter is not much less than that of Algol, be supposed to revolve about that star at the diftance of thirty-three diameters of Algol, in the given period of 2 days, 20 hours, 49 minutes, 3 feconds, in an orbit whose plane paffes at present through or near the Earth, it will caufe certain eclipses that will agree very well with the appearances observed. BOOK |