On the Sular Eclipse o/' 1 820. 25 



Again, in gases considerably lighter or heavier than ammo- 

 nia, as hydrogen or hydriodic acid, the lower the temperature 

 of operation generally the less v/ould be the influence of vapour ; 

 whilst in gases M-ith about the specific gravity of '6 it may 

 happen that diminishing the temperatm'e would augment the 

 error, J. Herapath. 



IV. On the Solar Eclipse isohich took place on September 7, 1 820. 

 By F. Baily, Esq. F.E.S. and L.S* 



Xhe solar eclipse of the 7th of September last having ex- 

 cited general attention throughout Europe, on account of its 

 magnitude, I shall venture to lay before the Society such ob- 

 servations as I myself made relative thereto, and also the re- 

 sult of such observations as have been communicated to me by 

 others, whose accuracy I have no reason to doubt. These 

 latter however are at present neither so numerous nor import- 

 ant as I had reason to expect, considering the number of good 

 observers, who must have witnessed this phasnomenon: never- 

 theless I flatter myself that the observations of such persons will 

 eventually be communicated to the pubhc in some other manner. 

 My own observations were made at Kentish Town, near 

 the bottom of Highgate Hill, in N. lat. 51° 33' 3i", and 

 W. long. 3 5", 2 in time, from Greenwich. The state of the 

 clock was determined by several altitudes of the sun, taken 

 on the morning and evening of the 6th, 7th, and 8th, with a 

 Troughton's reflecting circle ; the results of which agreed with 

 each other to great exactness. The following are the times of 

 the beginning and end of the eclipse. 



h , // 

 Beginning = 21 42,4"^ 



^"^ - ^ ^^ ^^'^ \ mean time at the place. 



Duration =2 51 58, 7 J 

 In noting the time of the beginning of the eclipse, I have not 

 made any allowance for the first second or two of time, which 

 must (I think) in all cases elapse before the commencement of 

 the eclipse can beccmie visible to a spectator, even with the best 

 telescopes. With res}iect to the termmation of the eclipse, I 

 do not consider any such allowance to be necessary, as the eye 

 can follow the moon till it is completely ofl'the sun's disc. The 

 telescope, made use oi; was a 3h feet refracting telescope by 

 Tnlley, with an object-glass of 3f inches diameter, and mag- 

 nifying 38 times: Inil tiie object end was covered with a brass 

 cap, which reduced liie aperture to two inches. The eye was 



• From Memoirs of the* Astronomical Society of London. 

 Yol 60. No. 29 1 . July 1 822. D protected 



