VOL. LXIV.] PHILOSOPHICAL TRANSACTIONS. 529 



4*" 14"" 364-% the difference of longitude deduced from the immersions. The 

 correction of the Nautical Almanac, by the mean of the 6 emersions, is — 16-^", 

 which applied to 4*' 14*" 45-^", the longitude of Jupiter's inlet found by the emer- 

 sions, by the help of the Nautical Almanac, gives 4^ 14™ 29% the longitude 

 deduced from the emersions. The mean of these 2 results, found from the im- 

 mersions and emersions separately, is 4*" 14'" 33* the proper difference of longi- 

 tude of Jupiter's inlet west of Greenwich. I have here made no allowance for 

 the difference of power of the 2-feet reflector, used at Jupiter's inlet, and the 

 6-feet reflector, used at Greenwich ; because the mean is taken between the re- 

 sults from the immersions and emersions; which method includes that correction; 

 that is to say, gives the same result whether that correction be made or not. 

 From the foregoing comparisons it should seem that the air is much clearer at 

 the island of Anticosti than at Greenwich, which Mr. Wright confirmed to me, 

 since the immersions give the longitude only 7^" greater than the emersions; 

 which shows that Mr. Wright observed an immersion only 4' sooner, and an 

 emersion as much later, with a 2-feet reflector, than was done at Greenwich in 

 a 6-feet reflector; though, in an equally good air, this latter telescope would 

 have had the advantage of the former by 20^, instead of 4'. 



XXFI. Extract of a Letter from Mr. Humphry Marshall, of JVest Bradford, 

 in Chester County, Pennsylvania, to Dr. Franklin, sent with Sketches of the 

 Solar Spots, dated May 3, 1773. p. I94. 



The appearances of these spots were not engraven. Mr. M. says he is of 

 opinion that the spots are near the sun's surface, if not closely adhering to it, 

 for these reasons : 1 . That their velocities are apparently greatest near the centre, 

 and gradually slower towards each limb. 2. That the shape of the spots varies, 

 according to their position on the several parts of the sun's disc; those that 

 appear broad, and nearly round, when on the middle, seeming, at their first 

 appearance on the eastern limb, but as lines; and, as they advance towards the 

 centre, become oval, then round, and, in their progress to the western limb, 

 appear again as ovals and lines. His other remarks were, that the spots were 

 124- days, and about 2 or 3 hours, in passing; that though some continued 

 visible from one limb to the other, a few would disappear, after having been 

 visible several days; and others divided into parts; that scarcely any spots ever 

 appeared beyond what may be called the polar circles of the sun ; and that the 

 same spot never appeared a 2d time, on the eastern limb, at least hot in the same 

 form and position. 



XXni. Jcconnt of the House-martin, or Martlet. By the Rev. Gilbert 



fVhite. p. 196. 

 Reprinted in Mr. White's History of Selbome. 



VOL. XIII. 3 Y 



