vox. LVIir.] VHILOSOPHICAL TRANSACTIONS. 583 



lucid point of the sun's surface to emit 150 particles in one second, which are 

 more than sufficient to give continual light to the eye, without the least appear- 

 ance of intermission; and then the particles, on account of their great velocity, 

 will be behind one another more than 1000 miles, and leave room enough for 

 others to pass in all directions. 



XLVl. Astronomical Observations made at Stvetzingen, in 1767 and I768; ea?n' 

 traded from several Letters written by Father Christian Mayer, F.R.S. p. 345. 



These observations are chiefly of the meridian altitudes of the sun and fixed 

 stars; with some observations of the eclipses of Jupiter's satellites : of no use now 

 to be retained in these Abridgments. 



XLVll. Observations of the Transit of Venus over the Sun, and the Eclipse of 

 the Sun, on June 3, 1769, made at the Royal Observatory. By the Rev, 

 N. Maskelyne, B.D., F.R.S. p. 355. 



The weather, which had been cloudy or rainy here, with a south wind, for 

 the greatest part of the day, began to clear up at 4 o'clock, in the afternoon, the 

 wind having returned to the west, the same quarter in which it had been the 

 afternoon before, which was remarkably fine and serene, though it changed early 

 in the morning preceding the transit. Towards the approach of Venus's ingress 

 on the sun, the sky was become again very serene, and so continued all the 

 evening, which afforded as favourable an observation of the transit here as could 

 well be expected, considering that the sun was only 7° 3' high at the external, 

 and 4° 33' at the internal contact. Mr. M. observed the external contact of 

 Venus at 7^ 10"" 58' apparent time, with an uncertainty seemingly not exceeding 

 5'; and the internal contact, by which he means the completion of the thread of 

 light between the circumferences of the sun and Venus, at 7*" 29"* 23' apparent 

 time, with a seeming uncertainty of only 3' ; for so long was the thread of light 

 in forming, or the sun's light in flowing round and filling up that part of his cir- 

 cumference wliich was obscured by Venus's exterior limb. Yet he would not 

 hence infer, that observations made by astronomers in distant places should agree 

 together within such narrow limits ; for he knows they will not even in the same 

 place, and that a difference in the skill or judgment of the observers, in the te- 

 lescopes, and perhaps in some other little circumstances, not easily distinguished, 

 may produce much greater disagreements, especially if the sun be low, as it was 

 here; in like manner as in observing the eclipses of Jupiter's satellites, the im- 

 mersion or emersion shall often seem instantaneous, or nearly so, equally to two 

 observers in distant places, and yet the absolute times of the observations may 

 differ a minute of time or more from each other, owing to the difference of te- 

 lescopes, weather^ or other circumstances. Indeed, in the present case, the 

 limit of differences is certainly much narrower ; but what it is he does not at 



