1880.] • Northern Sea-horizon. 357 



taken from 1860 to 1863, the barometer also varied but slightly, 

 from 29"53 to 29'70 in the first set, with a mean 29619; and 29'36 

 to 29'71, with a mean of 29"63, in the second set. The computed 

 refraction for the first set varied between 31' 58" and 33' Y, with 

 a mean of 32' 28"; for the second set between 32' 10" and 32' 49", 

 with a mean of 32' 14". The uniformity of these results is remark- 

 able; it need only be observed that for a zenith distance of 90" 21', 

 with the barometer at 30 inches and the thermometer at 50 F., 

 the normal refraction would be about, according to Ivory's theory, 

 39' 20", which by the low barometric ]3ressure and high tempera- 

 ture in the case in question, would be according to the same 

 writer reduced by about 4', to about 35' 20", or about 3' more than 

 actually observed. 



These observations taken by Sir A. Lang seem to be the most 

 striking examples of horizontal refraction that I have been able to 

 find in publications on Astronomy. But in Bessel's Fundmnenta 

 Astronomice, p. 53, there will be found seven observations of the star 

 Lyra, taken by Bradley at the Royal Observatory at Greenwich in 

 the years 1750 — 2, at a zenith distance of about 89" 30'. Accord- 

 ing to Bessel, the errors between the observed and theoretical 

 positions of the star was in one case nothing, and in only one more 

 than 1', when it was 1' 16". These last observations may perhaps 

 be thought by some to militate strongly against the accuracy of my 

 own observations, but, without entering on any other points, it 

 seems to me not unlikely that at very low altitudes the refraction 

 sustained by the huge disk of the Sun would not be the same as 

 that of a pointed ray (if I may be allowed the phrase) such as that 

 emanating from a fixed star : or, to speak more precisely, may it 

 not be possible that the wave undulations originating in so large a 

 volume of light ^,s that of the Sun, wdien transmitted through a 

 long series of strata of atmosphere perhaps unequally warmed, 

 may be affected by refraction very differently from those origi- 

 nating in a star, even of the first magnitude ? 



Brinkley and Plana have also given examples of stars observed 

 at low altitudes ; the former at an altitude of 2° 18', the latter 

 of 1° 36'; in which cases the observed and theoretical positions of 

 the star are said to have harmonized within a very few seconds; 

 but it will be seen that in neither of these cases is the altitude 

 nearly so small as in my own observations \ 



I wish it to be fully understood that I do not wish to throw 

 any doubt on the accuracy of the laws of refraction hitherto 

 adopted by astronomers and mathematicians down to an altitude 

 of about 3", or even lower. During the months of June and July 



^ For Brinkley's observations, see Transactions of the R. Irish Academy, 1820; 

 vol. XIII. p. 172. Plana's I have not been able to find, but they are referred to in 

 the Introduction to Shortrede's Logarithmic Tables, Introd. p. 12. 



