5 
with a Method of measuring the Dip at Sea. 
the month at 68°, the evening of that day (August 22, 1800) 
was unusually cold, the thermometer being 55°. The water 
might be supposed to retain the temperature it had acquired 
during a few weeks preceding, and, by warming the stratum of 
air immediately contiguous to it, might cause a diminution of 
its refractive density, sufficient to effect this inverted curvature 
of the rays of light, in the manner formerly explained. As I 
was at that time unprovided with instruments of any kind, I 
had it not in my power to estimate the quantity of refraction, or 
temperatures ; and can only say that, to my hand, the water felt 
in an uncommon degree warmer than the air. 
Being thus furnished with an unexpected field for observa- 
tion, I from that time took such opportunities as similar changes 
of the weather afforded me, of examining and measuring the 
quantities of refraction that might be discovered by the same 
means over another part of the river, that I found most suited 
to my convenience. 
The situation from which the greater part of my observations 
were made, was at the SE corner of Somerset house. The view 
from this spot extends under Blackfriars bridge, towards London 
bridge, upwards of a mile in length, and in the opposite direction 
through Westminster bridge, which is three quarters of a mile 
distant. 
Such distances are however by no means necessary ; and indeed 
the air over the river, in cold weather, is generally, or at least very 
frequently, not sufficiently clear for seeing distinctly to so great 
distances. For, since the winds which are most likely to effect 
a sufficient change of temperature, on account of their coldness, 
are usually from the E, or NE, the principal smoke of the town 
is then brought in that direction, and hovers, like a dense fog 
