magnitude to the fourth; its changes are confined, however, to a few hours, as it continues for rather more than 2 days 12 hours at its state of greatest brightness.
Stars have also occasionally appeared suddenly in various parts of the heavens, blazing forth for a time with extraordinary lustre, and after remaining awhile apparently immovable, have gradually decreased in brightness, and finally altogether vanished. These are properly termed temporary stars. Thus there suddenly appeared in the time of Tycho Brahe, (1572, 11th November), in the constellation of Cassiopeia, a most lustrous star, equalling Sirius in brightness; it continued increasing in brilliancy up to December, 1572, when it actually surpassed Jupiter and Venus when nearest to the earth, and was visible at mid-day. From this period forward it began to diminish rapidly, and in March, 1574, it had completely disappeared from the heavens. Another equally brilliant star burst forth on the 10th October, 1604, in the constellation of Serpentarius, and continued visible till October, 1605. The fact of the sudden appearance and subsequent disappearance of such temporary stars affords an irrefragable indication that there must exist also in space immense dark bodies, absolutely invisible to us, and of which accordingly we cannot possibly have any knowledge, as light is the only means of communication between the stars and the earth.
There remains now for us still to consider another
