Bobby $ishel, age 11, of New Providence, Tennessee, for his question:

How do they measure the depth of the oceans?

The deepest trench so far measured in the ocean floor dips 37,?82 feet below the waves. It is the Mindanao Trench of Mindanao Island, in the Philippines. Oceanographers, of course, do not probe such depths with a long stick or a tape measure. Instead of a solid measuring rod, they send dawn pulses of invisible sound energy. The instrument they use is called an echo sounder. It works because we know the speed at which sound travels through water and echoes back from the solid ocean floor below.

The first echo sounders were used soon after World War I and better ones were made with newer and still newer electronic gadgets. Nowadays the technique of using echoes to measure the depth of water is called "sonar." An instrument sends sound waves from a ship straight down to the solid ocean floor. There they bounce back up through the water. The two way sonar trip is timed to the fraction of a second and experts can figure the exact distance of the up and down journey. The depth of the ocean, of course, is just half of the two way trip.