Here is a better explanation of how the glider works, from the Science Notebook of today's the Washington Post:
Monday, February 11, 2008; Page A05 Motorless Sub Keeps Going Scientists seeking to gather temperature, salinity and other data from the oceans have long had two choices: steam out to sea on expensive research ships or launch unmanned submersibles whose batteries typically die in a few days. Now engineers and oceanographers have successfully tested a novel unmanned mini-sub that grabs energy from temperature differences in the ocean. In an ongoing test, the "thermal glider" has been traveling, without a propeller, for nearly two months. "We now believe the technology is stable enough to be used for science. It is no longer just a prototype," said Dave Fratantoni of the Woods Hole Oceanographic Institution on Cape Cod. Made by Webb Research Corp. of Falmouth, Mass., the glider changes its buoyancy by pumping fluid back and forth between bladders inside and outside its hull. Near the surface, where waters are relatively warm, wax within a chamber melts and expands, producing a pumplike force that can push water between bladders. To ascend from frigid depths, fluid is pumped from an inner bladder to one outside. The vessel's mass does not change, but its volume increases, increasing buoyancy. Back at the surface, pumps are recharged as wax melts and expands anew, even as fluid is drawn again to the inner bladder, reducing volume and slowly sinking the vessel again. Fixed fins convert the rising and falling into forward momentum, just as a paper airplane's wings make it glide forward when dropped. The six-foot craft travels about 1 mph, repeatedly bobbing up and then sinking to 4,000 feet as it goes, fueled by a temperature differential of about 43 degrees Fahrenheit. Instruments that can run on batteries for months gather data from the ocean and transmit to satellites with each surfacing. One goal is to study climate change. And because the glider has no motor, Fratantoni said, it is ideal for underwater acoustic studies. -- Rick Weiss
