off again, and plumping over-head in the uncomfortable
element; his intelligent countenance, in the meantime, wearing the
impress of despair. But Oscar soon helped him from his disagreeable
position. Finding himself on his legs again, he did not resume his
sport; but, shivering with cold, and dripping with water, almost at the
freezing point, and with his head hanging downward, and his tail
drooping between his legs, he started towards home--a wiser and a
sadder dog.

When Oscar got home, he found the family some what alarmed for his own
safety. Tiger had arrived some time before, and as it was evident that
he had been overboard, and as he was known to have gone off with his
master, Mrs. Preston felt some anxiety, not knowing but that both Oscar
and the dog had broken through the ice. But his arrival dispelled all
fears, and his account of Tiger's misfortune served to amuse the
children for the rest of the day. As for Tiger himself, he seemed
heartily ashamed of the part he had played, and could hardly be
persuaded to leave the chimney-corner for a moment, or even to look up,
when the children inquired for his health.

"I don't see what good air-holes do. I wonder if anybody knows what
they are for," exclaimed Ralph, as the children and their mother were
seated around the sitting-room table in the evening.

"They are traps set to catch skaters, I suppose," said Oscar.

"And dogs," added Ella.

"But don't you know what they are for, Alice?" continued Ralph.

"Yes," replied Alice, who had studied natural philosophy at school,
"they are the breathing holes of the fishes. Fishes can't live without
air, any better than we can; and a pond or river frozen over solid,
without any air-holes, would be as bad for them as a room from which
all fresh air was shut out would be to us. You can sometimes catch
fish very easily by cutting a hole in the ice, for if they feel the
need of air, they will rush right up to the opening."

"But how are the air-holes made?" inquired Ralph.

"I believe," replied Alice, "that they are generally made by springs
that bubble up from the bottom. These springs come from the earth, and
the water is so warm that it gradually thaws the ice over them. The
fish often finish the process by jumping up through the ice before it
has entirely melted. When the cold is very intense, and these springs
have frozen up, some of the water is absorbed by the earth, which
leaves a vacuum or empty space