•26 On the Sular Eclipse 



piolccleJ from llie rays of tlie sun by a dark glass of a red co- 

 lour ; a circumstance which I have thought proper to mention 

 in this place, as it appears, from the remarks of M. Messier, 

 that the colour of the ghiss is not immaterial in observations of 

 this kind. 



The sun was perfectly free from spots during the whole ot 

 the day; and had been so for the day or two previous thereto. 

 Soon after the commencement of the eclipse, a succession oi 

 flj-ing clouds prevented any correct measures being taken of 

 the enlightened part of the sun's disc. Towards the middle of 

 the eclipse however the clouds dispersed ; and I had an excel- 

 lent opportunity of measuring the diameter of the moon on the 

 sun's disc, with one of Troughton's spider-line micrometers, 

 attached to the telescope. By placing the two lines of die mi- 

 crometer as tangents to tlie moon's disc, I found that the di- 

 stance between them was 41-20 revolutions. But this was evi- 

 dently too great by the thickness of one of the lines, which I 

 found to be equal to five divisions: therefore the diameter of 

 the moon was only 4-1 "15 revolutions. The value of each re- 

 volution (by taking as a standard, the diameter of the sun on 

 tliat day, as given' by Uelambre's tables,) was 42",999: there- 

 fore the apparent diameter of the moon, in the direction in 

 which it was measured, was 29' 29'', 4-. But this direction was 

 inclined to the horizon about 75 degrees: which (on account 

 of the refraction) diminished the true diameter in that direc- 

 tion exactly 1",0: so that its apparent diameter, measured 

 horizontal]}-, would be 29' SO",^ ; and consequendy its semi- 

 diameter equal to 14' 45''',2. 



Now the horizontal semidiameter of the moon, at noon on 

 that day was, according to Burckhardt's tables, 14' 4l",02; to 

 which must be added 8", 71 for the augmentation at 2 o'clock 

 (the hour of observation): thus making the apparent semidia- 

 meter at that time equal to 14' 49",73, or 4",53 more than the 

 above observation. I would here remark that, according to 

 Burgh's tables, the semidiameter of the moon was 14' 43", 13: 

 which, allowing for * he augmentation, would make the apparent 

 semidiameter 6",64 more than the above observation. 



After the middle of the eclipse (the atmosphere remaining 

 beautifidly clear) I proceeded to measure the enlightened part 

 of the sun's disc, or the distance of the borders of the sun and 

 moon, with a telescope fitted up for the occasion by the Rev. 

 Dr. Pearson, with a Rochon's prismatic micrometer. It was 

 only 19 inches long, 1 inch diameter, and magnified about 

 30 times: but it was admii-ably adapted for the purpose in- 

 tended. The prism moved through the whole length of the 

 tube by means of a rack and pinion; and took in a scale of 



36', 



