2 Dr. Wollaston’s Observations on horizontal Refraction ; 
The definite reflecting surface which he supposes to take 
place between two strata of air of different density, is by no 
means consistent with that continued ascent of rarefied air 
which he himself admits; and the explanation founded on this 
hypothesis will not apply to other cases, which may all be 
satisfactorily accounted for, upon the supposition of a gradual 
change of density, and successive curvature of the rays of light 
by refraction. 
I have since learned that the same subject had also been ably 
treated by Mr. Woltman, in Gilbert’s Annalen der Physik ; 
but I have to regret that his dissertation, as well as that of 
Gruber, in the same Annals, were written in a language that 
was unknown to me, and that I could not avail myself of the 
assistance that I might otherwise have received from their 
researches. 
When I formerly engaged in this inquiry, being impressed 
with the advantage to be derived from it to nautical astronomy, 
on account of the variations in the dip of the apparent horizon, 
from which all observations of altitude at sea must necessarily 
be taken, I suggested the expediency of a series of observations, 
to be made by a person attentive to those changes of tempera- 
ture or moisture of the atmosphere, on which he might find the 
depression of his horizon principally to depend. I had at that 
time no expectation that I could myself pursue this subject 
farther to any useful purpose, having little prospect of residing 
for a sufficient length of time in view of the sea, and seeing no 
other method by which the same end might be accomplished. 
1 have, however, since that time, found means to satisfy myself, 
by observations over the surface of the Thames, that although 
the quantity of refraction varies in general with any change of 
