XVII. Some further Observations on Atmospherical Refraction, 
By Stephen Groombridge, Esq. F. R. S. 
Read March 31, 1814,. 
In my former paper on atmospherical refraction, communi- 
cated to the Royal Society by my late friend, Dr. Maskelyne, 
I considered the few observations made below 80® of zenith 
distance, as not sufficiently to be depended on, for the compu- 
tation of a general formula of refraction : and I therefore used 
ij Ursce Majoris (78° 10' zen. dis.) as the lowest star for that 
purpose. Having since applied the computed refraction from 
the formula thence obtained, to observations of stars below 80°, 
I have noticed, that such stars so corrected, appeared to be 
further from the zenitli below the Pole, than they ought to 
have been, from the observations above the Pole’: and there- 
fore that the refraction was less at those distances from the 
zenith, than I had assumed. Tliis has induced me, in the 
years 1811 and 1812, to make a course of observations of stars 
below the Pv le, above 80° zenith distance ; and as near to the 
N 
horizon, as the trees in Greenwich Park would permit; these 
being higher than the level of my Observatory. It may also 
be remarked, that those stars in my former table below 80°, 
produce the co-latitude in excess; as a confirmation, that 
the same formula will iv)t apply to those larger arcs, where, 
from the rapid increase of the tangents, a small error in the 
assumed quantity bjcomes more sensible. Although various 
hypotheses may be formed, from the known density and tern- 
