32 degrees of Fahrenheit's scale, it loses its liquid
form and becomes ice. A somewhat lower temperature than this is
necessary to freeze salt water; the reason being, that greater force is
required to expel the salt which the sea holds in solution,--which salt
is always more or less expelled in the process of freezing.

Ice commences to form in the shape of needles, which shoot out at angles
from each other. In smooth water, under the influence of intense cold,
the process is rapid, and a thin cake soon covers the water, and
increases in thickness hour by hour. But when the sea is agitated the
process is retarded, and the fine needles are broken up into what arctic
navigators call _sludge_. This, however, soon begins to cake, and is
broken by the swell into small cakes; which, as they thicken, again
unite, and are again broken up into larger masses. These masses, by
rubbing against each other, have their edges slightly rounded up, and in
this form receive the name of _pancake_ ice.

When a quantity of ice covers the ocean in a wide level sheet of
considerable extent, it is called an _ice-field_. Fields of this kind
are often seen by navigators hundreds of miles in extent, and nearly
thirty feet thick. Ice of such thickness, however, only shows five or
six feet above water. When fields are broken by heavy ocean-swells, the
edges are violently forced up, and fall in debris on the surface; thus
_hummocks_ or mounds are formed.

When field-ice breaks up under the influence of an ocean-swell, caused
by a storm, the results are terrific.

An exceedingly graphic account of an incident of this kind is given by
Dr Brown, in his "History of the Propagation of Christianity." He
writes:--

"The missionaries met a sledge with Esquimaux, turning in from the sea,
who threw out some hints that it might be as well for them to return.
After some time, their own Esquimaux hinted that there was a
ground-swell under the ice. It was then scarcely perceptible, except on
lying down and applying the ear close to the ice, when a hollow,
disagreeable, grating sound was heard ascending from the abyss. As the
motion of the sea under the ice had grown more perceptible, they became
alarmed, and began to think it prudent to keep close to the shore. The
ice also had fissures in many places, some of which formed chasms of one
or two feet; but as these are not uncommon in ice even in its best
state, and the dogs easily leap over them,