and Richter, have promised to continue their researches on the sub-
ject, some great and important fact must, he thinks, issue from their
labours.
Shield, from five Years’ Observations, exhibiting its proportional il- luminated Parts, and its I rregularities of Rotation ; with Conjectures respecting unenlightened heavenly Bodies. By Edward Pigott, Esq. In a Letter to the Right Hon. Sir Joseph Banks, K.B. P.R.S.
Read February 7, 1805. [Phil Trans. 1805.1). 131.]Mr. Pigott, some years ago, presented to the Royal Society a paper, which'is printed in the Philosophical Transactions for the year 1797, on the periodical changes of brightness of two fixed stars. The first part of the present paper consists of a series of observations made since those of the former paper, during the space of nearly five years, on one of the said stars, namely, that in Sobieski’s Shield. These observations are fully detailed in various tables; and mean results are deduced from the observations given in the former paper, and from those described in the present one. The results are as follows :—Rotation of the star on its axis, 62 days—Duration of brightness at its maximum, without any perceptible change, 9% days. -—Duration of brightness when it does not attain its usual brightness, 20 days—Duration of brightness at its minimum, Without any per- ceptible change, 9 days.—Ditto when it does not decrease so much as usual, 20 days—Decrease in time, from the middle of its full brightness to the middle of its least, 33 days—Increase of time, from the middle of its least brightness to the middle of its full, 29 days.—Extremes of its diEerent degrees of brightness, 5th to 9th magnitude—Mean of its usual variation, 5th to 6th magnitude.
In the second part of this paper Mr. Pigott proceeds to examine some of the other phenomena belonging to this star, particularly one which, he says, is common to most of the variables, and likewise in some degree to our sun, namely, that the times of their periodical returns of brightness are, in general, irregular. In hopes of making some discovery respecting the cause of these irregularities, or at least of assisting future astronomers to form some opinion respecting them, Mr. Pigott made a series of observations on the star here treated of, beginning in October 1795, and ending in October 1801. These observations are detailed at full length in two tables; and it appears from them. that the periodical returns of brightness are uncommonly fluctuating, and that the diEerences between the extremes are very considerable. Mr. Pigott then, by way of explanation, offers the following Opinions and inferences.
lst. That the bodies of the stars are dark and solid.
2ndly. That their real rotation on their axes is regular, following uniform impulses.
3rdly. That the surrounding medium does, at times, generate and
