In the days when the United States was still a very new country, many people in Europe longed for the freedom they were sure they could find here. One of them was Ferdinand Hassler, a young Swiss mathematician. Hassler was no seaman when he set out for the new world in a sailing ship. But luckily he did know a great deal about the stars. After the captain of his vessel collapsed in a terrific storm, Hassler was able to look at the stars and tell the seamen how to steer the ship.

The things Hassler knew about mathematics made it easy for him to navigate, but real troubles began when the ship came into Delaware Bay. The map of the bay was old and very inaccurate. Hassler could not tell whether the ship was in shallow water or deep water, except by watching the leadline day and night.

This last part of his adventure made young Hassler very angry because it was so unscientific. He realized that the safety of all ships depended on accurate maps, called charts, of the coasts and harbors. Soon after he landed he began to make plans for a survey of the whole American coast. He talked to President Jefferson who agreed with him, and Congress finally gave him the job. At last his good charts began to help save lives.

Today the United States Coast and Geodetic Survey carries on the work Hassler started. Using many ships and small boats with marvellous equipment, the scientific men of the sea go about their important and often dangerous work. It’s their job to map the earth that lies under the oceans, rivers and harbors. Here are some of the things they do along the Alaskan coast.

A survey ship moves through the water sending sound waves to the bottom of the ocean. A delicate machine records the echoes made when the sound waves bounce back from the deep valleys and the high mountain tops that lie beneath the surface. Scientists know how fast sound travels in water, so they can tell exactly how deep it is. But some of the peaks are narrow and sharp. Even the wonderful machines miss them. And so these very modern vessels must do something old-fashioned and simple. Two of them travel side by side with a long wire cable hanging between them. If the cable catches on a rock, the men know they have snagged a sunken mountain top.

Often, men on shore help the men on ships with their surveying. The instruments they use are so delicate that the warmth of direct sunlight would cause inaccuracies. One slight error might mean a shipwreck. So, even in Alaska, a surveyor works under an umbrella.

You might think that all the charts and maps should have been finished in the long years since President Jefferson’s time. But the work of making charts can never be finished. The coastline is always changing. Currents and tides, storms and floods shift millions of tons of sand near the coast. Earthquakes and volcanoes raise land or lower it. A place that was safe for ships yesterday may be dangerous today.

In Alaska, glaciers that run into the sea grow bigger or melt back. Sometimes these enormous rivers of ice push themselves out under the water. The little survey vessels mapping the ocean’s bottom have to sail over the sunken glaciers. There is always danger that, at any moment, a great mountain of fresh-water ice may break loose and rush toward the surface. When this happens, any vessel nearby is almost certain to be destroyed.

Seafaring men need to know about the tides when they enter or leave harbors. Tides are very different at different places along our enormous coastline, and they change from one day to another. The Coast and Geodetic Survey has worked out a wonderful way of telling ships about tides in advance. Every day, records pour into Washington from all along the seacoast. The figures they give are put into a fantastic “thinking machine,” together with other figures about the sun and moon which cause the tides. Electricity is turned on, and in no time the machine tells what tides will be like with ordinary weather tomorrow or even next month.

Men have come a long way since they first learned to float on a blown-up animal skin or a bundle of reeds. For thousands of years, they have been inventing new and better ways to travel across water. But the oceans have an enormous power and force. Science and seamen still have much to learn about the power which they must fight and make work for them—and which will always be exciting.