motors are limited in their use chiefly by the difficulty of supplying electricity. Compressed gas motors have been used on some vehicles experimentally. They are simple, efficient, safe, cheap, reliable and operate in a variety of conditions. One of the difficulties met when using gas motors is the cooling effect of expanding gas. These engines are limited by how quickly they absorb heat from their surroundings. The cooling effect can, however, double as air conditioning. Compressed gas motors also lose effectiveness with falling gas pressure. Ion thrusters are used on some satellites and spacecraft. They are only effective in a vacuum, which limits their use to spaceborne vehicles. Ion thrusters run primarily off electricity, but they also need a propellant such as caesium, or more recently xenon. Ion thrusters can achieve extremely high speeds and use little propellant; however they are power-hungry. Converting energy to work The mechanical energy that motors and engines produce must be converted to work by wheels, propellers, nozzles, or similar means. Aside from converting mechanical energy into motion, wheels allow a vehicle to roll along a surface and, with the exception of railed vehicles, to be steered. Wheels are ancient technology, with specimens being discovered from over 5000 years ago. Wheels are used in a plethora of vehicles, including motor vehicles, armoured personnel carriers, amphibious vehicles, airplanes, trains, skateboards and wheelbarrows. Nozzles are used in conjunction with almost all reaction engines. Vehicles using nozzles include jet aircraft, rockets and personal watercraft. While most nozzles take the shape of a cone or bell, some unorthodox designs have been created such as the aerospike. Some nozzles are intangible, such as the electromagnetic field nozzle of a vectored ion thruster. Continuous track is sometimes used instead of wheels to power land vehicles. Continuous track has the advantages of a larger contact area, easy repairs on small damage, and high maneuverability. Examples of vehicles using continuous track are tanks, snowmobiles and excavators. Two continuous tracks used together allow for steering. The largest vehicle in the world, the Bagger 288, is propelled by continuous tracks. Propellers (as well as screws, fans and rotors) are used to move through a fluid. Propellers have been used as toys since ancient times, however it was Leonardo da Vinci who devised what was one of the earliest propeller driven vehicles, the "aerial-screw". In 1661, Toogood & Hays adopted the screw for use as a