The scales, overlapping each other as they do, serve to protect the fish in its journey through watery ways, and their smooth, polished surface rendered slippery by a sort of natural oil, helps it to move quickly. We have imitated the scales of a fish in the way in which we arrange slates and tiles to keeps our houses dry. You know how the slates on the roof of your house overlap each other, so closely that no rain can get between them. When I tell you that there are said to be nine thousand different kinds of fish in all parts of the world, you will understand that even in a large aquarium you can see but few varieties. In England alone hundreds of fresh-water fishes are known, while those whose home is in the sea are much more numerous still. It has been found that if fresh-water fish is taken out of its natural element and put at once into the sea, it will die. But there are some fish, like the salmon, which live in the sea, but go up the rivers to lay their eggs, and then back again to their proper home; taking "change of air," as it were, but taking it gradually, and not plunging into a foreign country all at once. Some fishes are great travellers. I have heard that what is called a "shoal" of herrings consists of millions of fish, and takes up a place in the sea larger than the area of London. This fish takes its name from an old word which means an army; and the herring-army has to come a long, long march--if we so speak of a journey through "the paths of the seas"--before it, as it were, encamps near our shores. In winter the herrings are far away north, within the Arctic Circle, but in the spring they go south, travelling in shoals, six miles in length, and three or four in breadth. When one of these great shoals comes near our northern shores it divides, one part travelling west, the other east. It is in September that the herring fishing begins, and a busy time it is for the fishermen. The fish are always caught at night, and the darker the night the better chance there is of a good catch. When I was a child I used often to stand and watch the boats setting out about sunset, and many a time did I wish I might be of the party, for I thought no treat could be greater than to be allowed to stay out all night and see the nets full of shining fish drawn in over the sides of the boat. However, the fishermen are too wise to take children with them, for any noise frightens the herrings, so the fishing is done in silence, under the quiet stars. If you saw a herring-net taken in, you might forget yourself so far as to scream with delight at the sight of the fish flashing like silver, and bright with blue and purple hues which no painter could copy. But the rainbow colours, like those you see upon a soap bubble, are almost as soon gone; they will have lost their brilliancy before the boats come in, and the men begin to throw the fish on shore, and to count them.