Ron Jones, 13, of Hattiesburg, Miss., for his question:

HOW DEEP IS A FATHOM?

A fathom is a unit of length used to measure ropes or cables and the depths of water. One fathom equals six feet.

Navigators mark a rope in fathoms and drop it into the water to measure depth. Sailors of average height often measure fathoms roughly  by extending both arms and measuring the rope from finger tip to t finger tip.

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. The speed of sound in water is known.

Continuous sounds of this kind can be taken all during a voyage. The fathometer is more reliable in deep water than it is in shallow water.

The fathometer contains two parts: a submarine oscillator, 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 set to a depth indicator and a recorder near the bridge.