John Markworth, age 12, of Lowell, Mass., for his question:

WHAT IS A FATHOMETER?

A fathometer is an instrument used on ships to measure the depth of the water. It works by sending a sound down through the water to be echoed back from the bottom. Navigators can measure the depth below the ship by measuring the time it takes the sound to return.

A fathom is a unit of length used to measure the depths of water and also ropes or cables. One fathom equals six feet. Navigators mark a rope in fathoms and they can also measure the depth of water by dropping the rope into it.

Sailors of average height often measure fathoms roughly by extending both arms and measuring the rope from finger tip to finger tip.

A fathometer contains two parts: a submarine oscilator, which produces the sound, and a hydrophone echo receiver. They are kept in tanks of water at the bottom of the ship. The echo is amplified and sent to a depth indicator and a recorder near the bridge. The fathometer is less reliable in shallow water than in deep water.

During a voyage, continuous soundings can be made with the fathometer so the navigator will always be aware of the water's depth.