But an event happened, one day, that almost wholly cured Tiger of his fondness for this kind of sport.  He was gaily tripping over the ice, by the side of his young master, when the latter suddenly turned about, and Tiger, in his haste to follow him, slid directly into an air-hole.  This was probably the first time he had enjoyed so extensive a cold bath; and as he was not a water-dog, it is not surprising that he was terribly frightened.  His piteous cries brought Oscar to his relief, who could not help laughing at the sorry plight in which he found his half-drowned canine friend.  He was floundering and paddling about in the water, now lifting himself almost out, upon the edge of the ice, and now slipping 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 between the ice and the water; and then the ice gives way under the weight of the atmosphere, and air is admitted into the water beneath.”