180 



and Richter, have promised to continue their researches on the sub- 

 ject, some great and important fact must, he thinks, issue from their 

 labours. 



An Investigation of all the Changes of the variable Star in Sobieski's 

 Shield, from five Years' Observations, exhibiting its proportional il- 

 luminated Parts, and its Irregularities 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,^. 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 different 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 differences between the extremes are very 

 considerable. Mr. Pigott then, by way of explanation, offers the 

 following opinions and inferences. 



1st. 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 



