438 POLAR ICE. 
may be shown by experiment, that if the water in which they 
float had only the temperature of 42 deg. the mass of ice 
would lose the thickness of an inch every hour, or two feet in 
a day. Supposing the surface of the sea to be at 52 deg. the 
daily diminution of thickness would be doubled, and would 
therefore amount to four feet. An iceberg having 600 feet of 
total elevation, would hence, on this probable estimate, require 
150 days for its dissolution. But the melting of the ice would 
be greatly accelerated if the mass were impelled through the 
water by the action of winds. A velocity of only a mile in an 
hour would triple the ordinary effect. Hence, though large 
bodies of ice are often found near the banks of Newfoundland, 
they seldom advance farther, or pass beyond the 48th degree 
of latitude. Within the arctic regions those stupendous blocks 
remain, by their mere inertia, so fixed on the water as com- 
monly to serve for the mooring of vessels employed in the 
whale-fishery. In some cases, however, it is a necessary pre- 
caution to lengthen the cables and ride at some distance from 
the frozen cliffs ; because the fragments of ice, which the sea- 
men term calves, are frequently detached from the under part 
of the mass, and, darting upward, acquire such a velocity in 
their ascent that they would infallibly strike holes into the 
ship's bottom. 
The ice produced from salt water is whitish, porous, and 
almost opaque. It is so dense, from the quantity of strong 
brine enclosed in its substance, that, when floating in the sea, 
it projects only one-fiftieth part above the surface. The po- 
rous saline ice has a variable thickness, yet seldom exceeding 
six feet. But this same ice which, during the greater part of 
the year, covers. the arctic seas, is annually formed and de- 
stroyed ; a small portion of it only, and at certain seasons, es- 
caping the general wreck. The thaw commonly lasts about 
three months ; and during that time the heat of the solar rays, 
which, though oblique, yet act with unceasing energy, whe- 
ther applied directly or through the intervention of the air or 
the water, is sufficient for the dissolution of all the ice pro- 
duced in the course of the autumn, the winter and the spring. 
It may be proved, by experiment, that under the pole itself, 
the power of the sun at the solstice could, in the space of a 
week, melt a stratum of five inches of ice. We may hence 
fairly compute the annual effect to be sufficient for thawing 
to the depth of forty inches. It should likewise be observed 
that, owing to the prevailing haziness of the atmosphere in the 
northern latitudes, those singular cold emanations which al- 
