Don Hurley, age 14, of Butte, Mont., for his question:

WHAT IS A STIRLING ENGINE?

A Stirling engine is an experimental source of power that someday may be used in autos and boats. It runs more efficiently and produces less air pollution than most engines.

Robert Stirling, a Scottish minister, invented the engine in 1816. The engines have never come into general use because they cost more to build than other engines.

A typical Stirling engine has a sealed cylinder that contains either helium or hydrogen gas. The gas goes through a cycle of pressure changes by means of a process of alternate heating and cooling.

As the gas expands and contracts, it causes a power piston to move back and forth inside the cylinder. A rod connects the power piston to a crankshaft that converts the back and forth motion to the rotary motion of the drive shaft.