But there are times—and they occur very frequently in northern waters—when fogs which no light can penetrate envelop sea and coast, and that is the most dangerous of all times to an approaching ship. The only means by which a warning can be given, in such an emergency, is by sound. In many places bells are rung, but often the place to be avoided is so situated that the roar of the surf would drown a bell’s note, and then fog-horns are blown. These fog-horns are of a size so immense, and voices so stentorian, that it requires a steam engine to blow them, and they utter a booming, hollow blast, a dismal note as we hear it when we are safe on the land, but sweet to the anxious captain whose vessel is laboring through the gloom under close-reefed topsails, and uncertain of her exact position. One kind of these horns is very complicated in its structure, and screeches in a rough, broken blare, a note far-reaching beyond any smooth, whistling sound that could be made. This shriek is so hideous, so ear-splitting, when heard near at hand, that no name bad enough to express it could be found; so its inventors went to the other extreme, and called it a siren, after those most enchanting of sweet singers who tried to entice Ulysses out of his course. This name is opposite in a double sense, indeed, for the sirens of old lured sailors to wreck, while our siren hoarsely bids them keep off. Finally, buoys, which at first were simply tight casks, but now are usually made of boiler-iron, are anchored on small reefs, to which are hung bells, rung constantly by the tossing of their support; and on other reefs buoys are fixed having a hollow cap so arranged that when a big wave rushes over, it shuts in a body of air, under great and sudden pressure, which can only escape through a whistle in the top of the cap, uttering a long warning wail to tell its position. It is in such times as this that the pilot comes out strong. A pilot is a man who has made himself thoroughly acquainted with certain waters where navigation is dangerous, and who is licensed by some proper authority, after training and examination, to direct vessels in safety in entering harbors or passing through other intricate places. A ship-captain may be an excellent navigator, but he is not expected to know every rock and sandbar crouching under the waves, and all the twistings and turnings of the entrance and channel of a foreign harbor, especially as these channels are subject to constant change. In this country, indeed, although coasting-vessels may refuse a pilot, the law will not permit captains coming from or bound to a foreign port to do so; and if any accident happens when no pilot is aboard the insurance money will not be paid, and the ship’s officers may be punished. [Illustration: A SIREN RIGGED UPON A MERCHANT STEAMSHIP.]