16‘0 GREENLAND VOYAGE. 
of a mile. And in 1821, from the 11th of July 
until the 21st of August, an interval of forty-one 
days, we only had three entire days that were 
free from fog. During this interval, we naviga¬ 
ted a sea generally embarrassed with ice, included 
between the 78th and 67th parallels of latitude, 
and between the 3d and 15th degrees of west lon¬ 
gitude. 
This extraordinary prevalence of foggy weather 
in the polar seas, during the summer months, is 
a fact which, though well known, has not, that 1 
am aware of, been explained. The fogs to which 
the icy seas are subject, frequently rest on the 
surface of the water, and extend only perhaps to 
the height of 150 to 200 feet, the sky, in the ze¬ 
nith, being often perfectly clear, while objects 
near the horizon, or surface of the sea, cannot be 
discerned at the distance of 150 yards. In con¬ 
sidering this circumstance, it occurred to me, that 
the cause of these low fogs was probably a diffe¬ 
rence ol temperature between the upper and low¬ 
er parts of the foggy stratum; the cold, during 
such fogs, being perhaps greater at the surface 
than at considerable elevations. The inverse of 
this, indeed, is generally found to occur before the 
foggy season sets in, the temperature, on the ave¬ 
rage of a great number of observations, having 
been found to be two or three degrees lower at 
