TO THE BARRIER

over three hundred fathoms of water. There are many
such shelves of ice around the Antarctic Continent but
the Ross Barrier is the largest yet discovered and the
most astounding. To understand how these barriers
are formed, imagine a house with a sloping roof upon
which a winter's snow has accumulated as a thick
mantle of compacted ice. As the mass accumulates it
will begin to overhang at the eaves and presently, when
a certain weight has been attained, a huge slab of it
will slide down the roof and fall with a heavy thud upon
the flower-beds below. But now imagine that the house
stands up to its eaves in water. Then, as the mass on
the roof accumulates, it will float out upon the water to
form a shelf still joined to the roof but supported upon
the water. And every now and then, when a certain
breaking strain has been reached, slabs of it will break
free and float away. The gentler the slope of the roof
the larger the floating shelf will become and the longer
it will be before a breaking strain is reached which will
break off a piece of it. The larger, therefore, will be
the pieces which will eventually break free. • In many
places the Antarctic Continent, under its continuous
mantle of compacted ice many hundreds of feet thick,
slopes gently down to the sea and there, in just this way,
the ice-cap pushes outwards upon the water to form a
barrier. A barrier is thus really a glacier front, for as
more and more compacted ice forms inland the barrier
creeps gradually outwards on to the sea. Breaking
strains continually set free great slabs of it which float
away as those immense tabular flat-topped icebergs
which drift northwards until they reach warmer water
where they begin to melt, change their centre of gravity
in the process and, rolling over, assume bizarre and
fantastic shapes. The outline of a barrier face is on
this account never quite the same two years in succes-
sion. The Ross Barrier is the largest of all the known
barriers. It is four hundred miles long from side to