544 



PHILOSOPHICAL TKANSACTIONS. 



[anno 1784. 



XXII. On the Periods of the Changes of Light in the Star Algol. By John 



Goodricke, Esq. p. 287. 

 As Mr. G. was now able, by collating some of his late observations on Algol with 

 those given at p. 456, to determine with greater precision the periodical return 

 of its changes, he wished to add this as a kind of supplement to that account. 

 The method here pursued is by taking the intervals between accurate observa- 

 tions of Algol's least brightness or greatest diminution of light, made at long 

 distances of time from each other, and dividing those intervals by a certain num- 

 ber of revolutions. The reason for chusing long intervals is, that the number 

 of revolutions being greater, the errors of observation may be diminished : all 

 error cannot however as yet be excluded, but he thinks the period is now, by 

 the following calculation, ascertained within 10 or 15 seconds. 



Mean times of Algol's 

 least brightness. 

 m 

 25' 

 39 J 

 251 



25 

 14 



1783 

 Jan. 14 

 Oct 

 Jan. 



Nov. 14 

 Jan. 14 

 Nov. 17 

 Feb. 6 

 Oct. 25 

 Feb. 6 

 Nov. 14 

 Feb. 6 

 Nov. 17 

 Feb. 2fj 

 Oct. 25 

 Feb. 26 .9 

 Nov. 14 8 

 Feb. 26 9 

 Nov. 17 4 

 Jan. 31 14 

 Nov. 14 8 

 Mar. 21 8 

 Nov. 17 4 



17/ 

 25 1 

 52 J 

 151 



39 J 

 151 

 17/ 

 15? 



52 J 

 431 



39 J 

 43 I 



17) 



43 1 

 52 J 

 29? 



17) 

 36 I 

 52 J 



$• an interval of 99 revolutions, each of 2 

 106 



107 

 91 



98 

 99 



84 



91 



92 



100 



84 



Hence the period of Algol's variation is, on a mean, 



2 20 49 



It appears now, that the duration of the variation is about 8 hours. 



XX I J I. Experiments on the Terra Ponderosa, &c. By IVm. Withering, M. D. 



p. 293. 

 Sect. 1. — *Terra Ponderosa Aerala. — This substance was got out of a lead- 

 mine at Alston-Moor, in Cumberland. Its general appearance is not much un- 

 like that of a lump of alum ; but on a closer inspection it seems to be composed 

 of slender spiculas in close contact, but more or less diverging. It may be cut 



* Carbonate of Barytes. 



