You know, however, that you can row a boat by means of a single oar, if you work it from side to side at the stern. You will not travel very fast, partly because the oar is not very big, and partly because you are not very strong. But still the boat will move. Now if you look at the hinder feet of a seal, you will see that they are very broad, that they are set far back upon the body, and that, if necessary, they can be placed side by side together. Then think of the body of the seal as a live boat, and of these great broad feet as an oar worked from the stern, and you will be able to understand how the animal swims. It just places these feet side by side, and uses them in such a way that they act upon the water exactly as an oar does, while their strength is so great that they drive the body along very swiftly. HOW THEY ARE KEPT WARM But if the seal is a hot-blooded animal, how can it remain in the sea for days together without being chilled? If we go to the seaside, and wish to bathe, we are advised not to stay in the water for more than ten or fifteen minutes; and if we were to do so, we might be made seriously ill. Yet the seal can live for days, or even weeks, in the icy seas of the far north and yet never seem to suffer from the cold at all. How is this? Well, the fact is that, first of all, nature has supplied the seal with a kind of mackintosh, to keep it dry. This mackintosh, in most seals, is made of a double coat of fur. First there is an outer layer of long, stout hairs, almost like bristles; and underneath there is generally another layer of soft, close hairs--those which you see in a lady's sealskin jacket. And in order to keep the water from passing through it, this double coat of fur is kept constantly oiled. All over the surface of a seal's skin are thousands upon thousands of little holes, each of which opens into a tiny bag of oil, and this oil is constantly oozing out on to the fur. So, you see, the furry coat really does act like a mackintosh, for it quite prevents the seal from ever getting wet. When an animal lives in water which is often covered with ice, however, something more than a mackintosh is necessary in order to keep it warm; so under the mackintosh nature has provided the seal with a thick greatcoat. And this greatcoat is made of a substance much warmer than cloth, or even than fur. It is made of fat. Just under the skin, covering the whole of the body, is a layer of fat two or three inches thick. And this keeps the seal so warm that even when it is lying upon ice it never gets chilled in the least. FULLY FITTED FOR ITS HOME The nostrils and the ears of the seal are made in such a way that water cannot enter them when the animal is diving. They are furnished with little valves, which are so arranged that they close as soon as the water presses upon them. And the greater the pressure the more tightly they shut up, so that not the tiniest drop of water can ever enter them.