C >3 3 
II, The Bakerian Lecture. Observations upon an unusual hori- 
zontal Refraction of the Air ; with Remarks on the Varia- 
tions to zvhich the lower Parts of the Atmosphere are sometimes 
subject. By the Rev. S. Vince, A. M. F. R. S. and Plumian 
Professor of Astronomy and experimental Philosophy , in the 
University of Cambridge. 
Read November 15, 1798. 
T he uncertainty of the refraction of the air near the horizon 
has long been known to astronomers, the mean refraction va- 
rying by quantities which cannot be accounted for from the 
variations of the barometer and thermometer; on which ac- 
count, altitudes of the heavenly bodies which are not more 
than 5 0 or 6°, ought never to be made use of when any 
consequences are to be deduced from them. The cause of 
this uncertainty is probably the great quantities of gross 
vapours, and exhalations of various kinds, which are sus- 
pended in the air near to the earth's surface, and the variations 
to which they are subject ; causes, of which we have no instru- 
ments to measure the effects which they produce, in refracting 
the rays of light. In general, the course of a ray passing 
through the atmosphere, is that of a curve which is concave 
towards the earth, the effect of which is to give an apparent 
elevation to the object ; and thus the heavenly bodies appear 
above the horizon, when they are actually below it ; but it will 
not alter the position of their parts, in respect to the horizon,. 
