The US company iRobot made a new kind of sub-marine  , called the Seaglider, last week.     The robotic submarine was developed by researchers at the University of Washington. It is about 1.8 meters long and weighs about 52 kilograms. It moves through water at a speed of up to 20-25 kilometers a day and can stay at sea for up to six months before needing a battery recharged. This special character results from its special propulsion system  . Unlike a traditional submarine, the Seaglider does not have a propeller  . It has an air tank that is emptied and filled to adjust   depth, as well as a pair of wings that help to guide and lead the submarine.     A traditional submarine, on the other hand, "puts water inside tanks to make it heavier, or it will expel   the water to make it lighter," explained Frederick, a professor at the University of Washington.     Each time the Seaglider comes to the surface, it sends the data which it collects to its controllers by satellite. Operators can also send the submarine new information. For example, they can change its course in their office through giving the computer orders.     Seagliders are already being used for scientific research, such as environmental studies. iRobot also hopes to sell the submarine to the military  . At $100,000 each, the Seaglider is a cheap way to keep an eye on the ocean.     "These things can go out there and monitor   large areas of ocean because they're very persistent. They can go for thousands of miles on their own. At the same time, you can tell them to stay in one place and they can circle," says Helen Greiner, chairwoman of iRobot. "This is science fiction stuff in many ways, but it's really out there and working." When the Seaglider comes to the surface, it will   _  .
change its battery send the data to its controllers force out a lot of water stay in one place