"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."
"Well, I 'm glad air-holes are good for something," said Oscar; "they 're troublesome enough to skaters. Jim Anderson skated right into one the other day, and came pretty near getting drowned. But I always keep my eyes open for them. I never got into one yet."
"You cannot be too careful when you are on the ice," remarked Mrs. Preston. "I felt so uneasy, that I was just going to send Ralph in search of you, when you got home."
After that day it required considerable coaxing to induce Tiger to go upon the boys' skating-ground. He manifested a decided preference to remain upon the shore, and look on; and when he did venture to accompany his master, he kept close by his side, and travelled over the treacherous ice with a degree of circumspection, which said very plainly, "You won't catch me in that scrape again, master Oscar!"