20 Dr J. B. Pearson, On the experiments made by [Nov. 22, 



upon a memoir I presented to the Society last spring {Proceedings, 

 Vol. in. pp. 352 — 8), describing some observations of the Sun on 

 the northern horizon, made by me last summer at the North Cape. 

 At the commencement of this century, the question seems to have 

 excited some interest : there being papers on the subject by 

 Mr Huddart, Prof. Vince, and Mr Wollaston in the Philosophical 

 Transactions for 1797, 1799, and 1803. As these memoirs how- 

 ever do not seem to me to contain any precise results, although 

 they record several interesting natural phenomena, I shall not 

 trouble my readers with an abstract of them. The investigations 

 however, of M. Biot, M. Le Gentil, and in a less degree those of 

 M. Bouguer in Peru, seem to me so relevant to the question of my 

 own observations, that I have thought that an abstract of them 

 may be of interest to science. 



It must be premised that the altitude of the Sun or a star 

 when close to the sea-horizon involves two things : the apparent 

 depression of the horizon itself, or the dip, technically so called, 

 and the atmospheric refraction. M. Biot's experiments are con- 

 fined to the former ; those of the two other savans mainly to the 

 latter : while my own observations, taken on the sea-horizon from 

 the deck of a ship at a height of about 18 feet, will evidently be 

 affected by both. 



Having premised this, I subjoin first an abstract of M. Biot's 

 observations, which were taken by him with the aid of M. Mathieu 

 at Dunkirk during the winter of 1809 — 10, and were published by 

 him at Paris in a separate volume in the following year. They 

 were taken with a graduated quarter-circle with telescope attached 

 and adjusted by a spirit level, and from stations at different alti- 

 tudes above the level of the sea : viz. (1) actually on the sand : 

 (2) a timber staging : (3) different stories of a house on the 

 beach : and lastly, from the top of the tower of the church. The 

 annexed Table shews the place of observation, and its elevation in 

 feet, the date, the temperatures of the sea and air on Fahrenheit's 

 scale, and the amount by which the observed result varied from 

 that which would be expected on theoretical grounds, a negative 

 sign (— ) being affixed where the depression of the horizon was 

 greater than the normal, and a positive one (+) when it is less. By 

 the normal or theoretical result is meant that given by the theory 

 explained at length by M. Biot in the same volume. I am unable 

 to say how far his theory at that time coincides with the views on 

 the subject laid by him before the French Academy in several 

 papers in the years 1854 and 1855. He then allowed that the 

 system generally adopted was that of Bessel, and spoke with 

 approval of the abstract of it issued by the Greenwich Observatory 

 in 1853 : but he was evidently dissatisfied with Bessel's method, as 



