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.
