VOL. LXX.] PHILOSOPHICAL TKANSACTIONS. (ggi 



increased. Nov. 2, 1779, the lustre of the periodical star is still increased. The 

 body is very full and round in the telescope. I magnified it 449 times very 

 distinctly, the evening being so fine; but my usual power is only 222. Nov. 

 20, 1779, the periodical star seemed to be as bright as before, but no brighter. 

 Nov. 30, 1779, " Ceti is considerably decreased. Its magnitude is less than (3 but 

 greater than a. December 4, 1779, the lustre of Ceti is only equal to a.. I 

 measured the distance of this star from the obscure one which follows it: first 

 measure l' 53".437; second measure l' 50".625. 



The weather was so cold, that I could hardly finish this last measure; but I 

 believe it to be too small. The disagreement is owing to the difficulty, and not 

 to want of accuracy in the micrometer. January 4, 1780, the periodical star is 

 very much diminished. Feb. 7, 178O, the periodical star was invisible to the 

 naked eye. 



Maupertuis accounts for the periodical appearances of changeable stars, by 

 supposing that they may be of a flat form, like Saturn's Ring, which becomes 

 invisible when the edge is presented to us. As tiie periodical star in collo Ceti 

 appeared always full and round when I viewed it with a telescope, this migiit at 

 first appear to contradict the supposition of Maupertuis; but, on proper con- 

 sideration, will be found not to be at all against it: for suppose the real apparent 

 diameter of this star to be J- of a degree ; then since it appeared to me at least of 1 ", 

 when at the full, it will follow, that there was an aberration, whatever might be 

 the cause of it, which amounted to 59'", by which its real apparent diameter 

 was increased from l'" to \". Now if this star, in one certain position, should 

 present its circular disc of \"' in diameter, and in another situation only its flat 

 edge which would appear as a line of \"' in length, both appearances, with the 

 aberration included, will still remain of a circular form: for adding the aberra- 

 tion 59'" to the length l'", the whole becomes \"; and adding 59'" to scarce any 

 breadth at all, the whole breadth will also become nearly I". 



Keill says, " It is probable, that the greatest part of this star is covered with 

 spots and dark bodies, some part thereof remaining lucid; and while it turns 

 about its axis, sometimes shows its bright part, sometimes it turns its dark side 

 to us; but the very spots themselves of this star are liable to changes, for it does 

 not every year appear with the same lustre. Sometimes it resembles a star of the 

 2d magnitude; in other years it can scarcely be reckoned among stars of the 3d 

 order; nor are the times of its visiting us always of the same duration." 



XXII. An Account of a new and cheap Method of preparing Pot-ash, with 

 Observations. By Thomas Percival, F.R.S., andA.S. p. 343. 



Reprinted in Dr. P.'s collected works, edited by his son. 



XXI II. On the Degree of Salubrity of the common Air at Sea, compared with 



4 T 2 



