These devices are propelled by air. Compressors, either electric or gasoline-powered, generate air pressure. Pneumatic tools include air hammers and pneumatic nailers. Fluid pressure drives these gadgets. The tool receives its power from a hose connected to the compressor. Air compressors can be electric or gasoline driven. Electric motors are more efficient than outboard engines, but they also are more expensive to purchase and operate. Gasoline engines are less efficient, but they are cheaper to buy and run. Most manufacturers recommend an electric motor as part of any automatic back-up system because it will continue to run the tool even if there is a leak in the gas line. This would allow you to finish your project without having to stop to repair the gas line. The type of tool you need depends on what kind of job you need to do. If you only need to drive nails, then an air hammer is enough. These tools use a heavy steel ball called a quill that is thrown at high speed against the metal head of the nail using compressed air. The force of the blow opens up small holes in the head of the nail through which more air can flow when the tool is recharged. This process continues until the nail pops into the hole cleanly.