The light buoys are more complicated and more diverse. There are more than a dozen different sizes and shapes, and the fuel is usually compressed oil gas or compressed acetylene gas. The buoys themselves—that is, the floats—may be of almost any shape. Some are spherical, some cylindrical. Some are long and thin, and others short and fat, but each one has a framework or a shaft of steel extending from ten to twenty feet above it. At the top of this the light is fixed, while the body of the buoy holds the gas. These lights flash intermittently, the gas, which is under pressure, operating a valve while a tiny “pilot light” in the burner remains always burning in order to ignite the gas when it is turned on to cause each flash. Some of these buoys carry a supply of fuel great enough to last for three months, and during that time they flash their lights every few seconds without fail, marking a danger or a channel, and are visible, sometimes, from distances of several miles. Thus the dangers of the sea are marked by lighthouses, lightships, and buoys, while harbour entrances and channels are marked as well. This has been done in order to save life and property and in order to expedite the passages of ships. No more do captains have to depend on guess and luck. Their accurate sextants and chronometers tell them where they are on the trackless sea. Their barometers tell them of approaching storms. Their compasses tell them their directions. And men ashore have built great lights on wave-washed rocks and surf-pounded beaches, on mighty headlands and shoals of sand. Lightships mark the treacherous spots where lighthouses cannot be erected, and mark, as well, the entrances to many harbours around the world. And once past these the mariner is led into the shelter of the harbour between long lines of buoys, each telling him its message, each aiding him on his way. He rounds a rock in mid-channel unscathed, because a buoy anchored there tells him how to turn. He finds his anchorage because of other buoys, and perhaps he makes his ship fast to still another, and knows that once more the ocean has been crossed in safety and his voyage is ended. Almost the whole of the surfaces of all the lands of earth bear the marks of man. Most people live their lives ashore amid nature that has been radically changed by man. Cities have been built, railroads flung across the land. Farms flourish and ploughs have turned up every inch of all their acres. A hundred years ago America was wild from the Alleghanies to the Pacific. Now one cannot cross it and be for more than a few minutes out of sight of signs of men. But the ocean rolls ever on just as it rolled in prehistoric times. No mark that man has made has changed the sea. Yet, while man is unable to change one single thing about its solitary waste, he has marked its greater perils and has conquered it. The perils of the sea are growing ever less, and ships owe much of this to the lights that mark its danger spots.