ATMOSPHERIC REFRACTIONS. 171 
long time, a great difficulty with me. At the 
very moment when the ice and other objects about 
the horizon were evidently raised several minutes 
of altitude, by being seen through the highly re¬ 
fractive medium, the effect of which has been so 
fully stated, the upper parts of the land were 
sometimes totally unaffected by it, remaining at 
their proper level, while the lower parts were in¬ 
tercepted by so much, as the horizon seemed to be 
elevated above its real position. As such, it ap¬ 
peared, that the influence which caused the loom¬ 
ing of the ice, by some peculiarity or other, did 
not extend to the land. It seemed to intimate, 
either that the looming of the ice was owing to 
reflection rather than refraction, or, that the 
tops of the mountains were above the vapour, 
and my position at the mast-head (the place 
from whence most of my observations were made) 
also above it, so that no influence could be ex- 
perficial strata of water will give out their heat, so as to have 
an increase of density from the surface to a certain depth 
below it” Through the medium thus constituted, many of 
the phenomena of unusual refraction may be seen in the 
most beautiful and satisfactory manner, though the diffe¬ 
rence of density in the medium employed, is only that pro¬ 
duced by the agency of heat alone. (See Edinburgh Ency¬ 
clopedia, article Heal, vo). x. p. 675.; and a valuable trea¬ 
tise on Optics, in the same Work, vol. xv. p. 617-—620.) 
