The next year, in March, 1672, it was seen again, but not exceeding the sixth magnitude ; since when it has been no further visible, though we have frequently sought for its return; its place is 9' 3° 17' a Inia* , and has lat. north 47° 28'. The sixth and last is that discovered by Mr. G. Kirch in the year 1686, and its pe riod determined to be of 41.14.i days ; and though it rarely exceeds the fifth Magni tude, yet is very regular in its returns, as we found in the year 1714. Since then we have watched, as the absence of the moon and clearness of the weather would permit, to watch the first beginning of its appearance in a six feet tube, that, bearing a very great aperture, discovers most minute stars. And on June 15 last, it was first perceived like one of the very least telescopical stars ; but in the rest of that month and July, it gradually increas ed, so as to become in August visible tc the naked eye, and so continued till the month of September. After that, it again died away by degrees ; and, on the 8th of December, at night, was scarcely discern ible by the tube ; and, as near as could be guessed, equal to what it was at its first appearance on June 25, so that this year it has been seen in all near six months, which is but little less than half its pe riod ; and the middle, and consequently the greatest brightness, falls about the 10th of September." The late improvements in astronomy, and particularly those in the construction of telescopes, have now given astrono mers an opportunity of observing the changes which take place among the stars with much greater accuracy than could be formerly done. Ina paper in the 76th volume of the Philosophical Trans actions, Mr. Edward Pigot gives a disser tation on the stars suspected by the astro nomers of the last century to be change able. For the greater accuracy in the in vestigation of his subject, he divides them into two classes ; one containing those which are undoubtedly changeable, and the other those which are only suspected to be so. The former contains a list of 12 stars, from the first to the fourth mag nitudes, including the new one which ap peared in Cassiopeia in 1572, and that in Serpentarius in 1604: the other contains the names of 38 stars, of all magnitudes, from the first to the seventh. He is of
opinion, that the celebrated new star in Cassiopeia is a periodical one, and that it returns once in 150 years. Keill is of the same opinion ; and Mr. Pigot thinks, that its not being observed at the expiration of each period is no argument against the truth of that opinion ; " since (says he) perhaps, as with most of the variables, it may, at different periods, have different degrees of lustre, so as sometimes only to increase to the ninth magnitude ; and if this should be the case, its period is probably much shorter." For this reason, in September, 1782, he took a plan of the small stars near the place where it for merly appeared ; but in four years had ob served no alteration. The star in the neck of the Whale had also been exa mined by Mr. Pigot, from the end of 1782 to 1786, but he never found it exceed the sixth magnitude ; though Mr. Goodricke had observed it on the ninth of August to be of the second magnitude, and on the third of September, the same year, it was of the third magnitude. Mr. Pigot de duced its period from its apparent equa lity with a smaller star in the neighbour hood, and thence found it to be 520,328, and 337 days. The most remarkable of these changeable stars is that called Al gol, in the head of Medusa. It had long been known to be variable ; but its period was first ascertained by Mr. Goodricke, of York, who began to observe it in the beginning of 1783. It changes continu ally from the first to the fourth magni tude ; and the time taken up from its greatest diminution to its least is found, at a mean, to he 2d Oh 49m and 3'. During four hours it gradually diminishes in lus tre, which it recovers during the succeed ing four hours ; and in the remaining part of the period, it invariably preserves its greatest lustre, and after the expiration of the term its diminution again com mences, According to Mr. Pigot, the degree of brightness of this star, when at its minimum, is variable in different pe riods, and he is of the same opinion with regard to its brightness when at its full ; butwhether these differences return re gularly or not has not been determined,