VOL. LIV.] PHILOSOPHICAL TRANSACTIONS. , }63 



but somewhat jagged and unequal. From 9*' to Q*" 15™ it exhibited a most vivid 

 resplendent whiteness, such as he believes was hardly ever observed before. 

 During that term, the phenomenon seemed altogether fixed and permanent, 

 without increase or diminution, without any apparent motion of the whole, and 

 indeed almost without the least external variation. An internal undulating 

 motion of the particles, constituting the white luminous matter of the arch, 

 was however discernible, from the first to the last moment .of its existence. 

 No stars were visible through the vapour itself, but 2 or 3 appeared at a small 

 distance from it. Not the faintest traces of a proper aurora borealis, either 

 before the first appearance, or during the continuance, or after the extinction of 

 the meteor, were to be seen. A little past 9 o'clock, the extremities of the arch 

 became faint, as did soon after the whole body of the luminous vapour itself. 

 About 9" 20"* the summit, or highest part of the arch, a few degrees to the n. 

 of the zenith, only remained ; which continued gradually decreasing till 9'' 27™, 

 when the whole totally disappeared. 



Mr. S. has not been able to meet with an instance of a similar phenomenon 

 in any physiological papers, published before the year 1750. But accounts of 2 

 or 3 meteors somewhat resembling that above described in our Philos. Trans.* 

 then occurred. However, this of the 23d of April, 1764, differed from one of 

 these in its extent, as well as the inconsiderable breadth of the zone forming the 

 arch, and the bisection of the hemisphere. From the others it was sufficiently 

 distinguished by its most vivid resplendent whiteness, without any short, white, 

 vibrating columns attached to it; especially, as it was neither preceded, 

 attended, nor followed by any streaming luminous rays or coruscations. 



LiTI. On the Equation of Time and, the True Manner of Computing it. Btf 

 Nevil Mashelyne, A. M, F. R. S. p. 336. 



M. Delalande says, in the Connoissance for 1760, which he repeats in the 

 publications of other years, that, " to calculate exactly the difference between a 

 mean and true time (that is to say the equation of time) at the instant of appa- 

 rent noon, the sum of the equation of the sun's centre, the difference between 

 his longitude and right ascension, the lunar equation, the equations of Jupiter 

 and Venus, and that of the precession of the equinoxes, with their proper signs, 

 must be converted into mean solar time. He adds, that it was impossible, before 

 this time, to obtain the equation of time exactly; 1st, because hitherto no ac- 

 count has been made of the four little equations, the sum of which may pro- 

 duce above 3 seconds of time ; 2dly, because it has been the practice to convert 

 the equation of the sun's centre, and the difference between his right ascension 



• Philos. Trans, vol. xlvi, p. 345, 346, 347, 6'48, 649. 

 Y 2 



