8 Dr. Wollaston’s Observations on horizontal Refraction ; 
that its quantity will be greatly influenced, and in general 
diminished, by dryness of the atmosphere. 
It appears, however, that no observable regularity is deducible 
from the measures above given ; but that the quantity, on some 
occasions, is far different from what the states of the thermo- 
meter and hygrometer would indicate. On the 9th of September, 
for instance, the difference of temperature is only and the 
evaporation, to counteract this slight excess of warmth, pro- 
duced as much as 3 0 of cold ; nevertheless, the refraction visible 
was full f . In this observation I think that I could not be 
mistaken, as the water was at the time perfectly calm, the air 
uncommonly clear, and I had leisure to pay particular attention 
to so unforeseen an occurrence. 
This one instance appears conformable to the opinion enter- 
tained by Mr. Huddart, and by M. Monge, that, under some 
circumstances, the solution of water in the atmosphere causes a 
decrease in its refractive power; but, on no other occasion 
have I been induced to draw a similar inference. 
The object that I have at all times chosen, as shewing best 
the quantity of refraction, has been either an oar dipped in the 
water at the greatest discernible distance, or some other line 
equally inclined ; and the angle measured has been, from the 
point where the inverted image is terminated by the water, to 
that part of the oar itself which appears to be directly above it. 
(The apparent magnitude of ec , Fig. p. 3.) 
The eight first angles were taken with a mother-of-pearl 
micrometer in the principal focus of my telescope, and are not 
so much to be depended upon for accuracy as the succeeding 
eight. These last were measured with a divided eye-glass 
