530 I'HILOSOPHICAL TKANSACTIONS. [aNNO 1754. 



XCl. Of a Fire-ball, seen at Hornsey. By H^tlliam Hirst, F. R.S. p. 773. 

 This phenomenon Mr. H. saw on Feb. l6, \75A, about 5 minutes before 11 

 at night. He was then going down the hill adjoining to the south side of 

 Hornsey church, and was not a little surprised to find himself suddenly sur- 

 rounded with a light equal to that of the full moon, though the moon, which 

 was theji 4 days old, had been set about 30 minutes. He had a distinct, though 

 short, view of a ball of fire, which at first appeared to be about 15° high, w. 

 by N. Its descent was not exactly perpendicular, but made an angle of about 80° 

 with the s. s. w. part of the horizon, moving from left to right, so that when it 

 went below the horizon, its bearing from him was w. n. w. It moved with great 

 velocity, not continuing visible much longer than 2 seconds; though he did not 

 lose sight of it till it descended below the horizon. But short as this duration 

 was, its shape might be well discerned. The diameter of the nucleus, or head 

 pf the meteor, appeared to be equal to the semidiameter of the meridional full 

 moon, and the tail, which terminated in a point, seemed not longer than twice 

 the diameter of the nucleus. And its track in descending seemed to be but 

 about 10° from the vertical; as the position in fig. 8, pi. 11. This meteor was 

 not attended with any noise, nor left any luminous stream after its descent below 

 the horizon. The appearance of such meteors at that cold season of the year is 

 the more extraordinary, as their generation is attributed to exhalations caused 

 by heat, or the action of the sun ; for which reason they are generally seen after 

 hot sultry weather. 



By the distinctness and red fiery colour of this phenomenon, he imagined that 

 it was not very high in the atmosphere; but should be induced to think other- 

 wise, if credit is given to an account from Dublin, which states that a like meteor 

 was seen there between 10 and 11 that night, which illuminated the whole 

 hemisphere, and continued about 4 seconds. The near agreement in these two 

 accounts, as to the situation and time (allowing for the difl^erence between the 

 meridians of Dublin and Hornsey), it being nearly half an hour past 10 at 

 Dublin when 1 1 here, makes it very probable that it was one and the same me- 

 teor; which, if so, is a proof that its height in the atmosphere must be very 

 considerable. 



XCII. A Comparison between the Notions of M. de Courtivron and Mr. Melvil, 

 concerning the Difference of Rcfrangibility of the Rays of Light. By Mons. 

 Clairaut, Memb. of the R. Acad, of Sciences at Paris, and F.R.S. p. 776. 



M. Clairaut observes that both the above-named gentlemen, Mr. Melvil and 

 Mr. de Courtivron (the former in a paper lately printed in these Transactions, 

 and the latter in a book published by him in 1752) had thought of accounting 



