274 
GREENLAND VOYAGE. 
ing appearance of the kind that occurred, the de¬ 
scription has been reserved for this occasion. In 
the following remarks on corona 1 , &c. I have taken 
advantage of my former observations on the sub¬ 
ject ; and instead of confining myself to a parti¬ 
cular description of what occurred on this day 
only, I have preferred giving a more general ac¬ 
count of the phenomena. 
Coronal may be observed whenever the sun 
shines during foggy weather; and this is often 
the case, the polar fogs frequently consisting mere¬ 
ly of a thin stratum, partaking of the nature of 
clouds, resting on the surface of the sea, and ex¬ 
tending only to the height of fifty or sixty yards. 
At such times, though objects ou the water can 
scarcely be discerned at the distance of a hundred 
yards, sometimes not more than fifty, the sun is 
frequently not only visible, but shines with near¬ 
ly the effulgence it exhibits in a clear sky. Under 
such circumstances, an observer, elevated on a 
ship’s mast, ninety or a hundred feet above the 
level of the sea, will perceive one or more prisma¬ 
tic circles depicted on the fog. These circles are 
all concentric, the centre being in a line drawn 
from the sun through the point of vision, until it 
meets the visible vapour in a situation 180° dis¬ 
tant from, or exactly opposite to, the sun. The 
number of corona; varies from one to four or five. 
