"By keeping it warm, on the same principle that your bed-clothes kept you warm last night. Heat passes very slowly through ice-that is, it is a poor conductor. With the snow it is the winter wrap of nature, which protects all life beneath it. When our ponds and rivers are once frozen over, the latent heat in the water beneath can escape through the ice but very gradually, and every particle of ice that forms gives out into the water next to it one hundred and forty degrees of heat. Were it not for these facts our ponds would soon become solid. But to return to the tub of water in the flower-room. The water, when placed there, was probably warmer than the air, and so would give out or radiate its heat until a thermometer, placed either in the room or in the water, would mark thirty-two degrees above zero. At this point the water would begin to freeze, but plants or vegetables would not. They would require slightly severer cold to affect them. But as soon as the water begins to freeze it also gradually gives out its latent heat, and before a particle of ice can form it must give out one hundred and forty degrees of heat to the air and water around it. Therefore the freezing process goes on slowly, and both the air and water are kept comparatively warm. After a time, however, the ice becomes so thick over the surface that the freezing goes on more and more slowly, because the latent heat in the unfrozen water cannot readily escape through the ice. It is therefore retained, just as the latent heat in the water of an ice-covered pond is retained." "It follows, then," said Leonard, "that after the water beneath the ice in the tub began to freeze slowly, the flower-room, in that same degree, began to grow cold." "Certainly, for only as the water freezes can it give out its latent heat. The thick wooden side of the tub is a poor conductor; the ice that has formed over the surface is even a worse, and so the water within is shielded from the cold. It therefore almost ceases to freeze, and so becomes of no practical use. An intelligent understanding of these principles is of great practical value. If I could have waked up and placed another tub of water in the room at two or three o'clock, or else taken all of the ice out of the first one, the process of freezing and giving out heat would have gone on rapidly again, and none of the plants would have suffered. I have heard people say that putting water in a cellar was all a humbug--that the water froze and the vegetables also. Of course the vegetables froze after the water congealed, or the cellar may have been so defective that both froze at the same time. The latent heat given out by a small amount of freezing water cannot counteract any great severity of frost." "The more water you have, then, the better?" said his father. "Yes, for then there is more to freeze, and the effect is more gradual and lasting."