I want to compress air using the weight of a tank of water. Under the tank I propose some form of positive-displacement cylinder -perhaps a ram or spring return type cylinder. The tank will be supported in such a way so that it will move along with the cylinder. When filling up the tank the weight should cause the cylinder to compress the air. When I drain some of the water the tank will recede to its original position and draw in more air until the tank is refilled. My question is: What type of cylinder is best for this? And can you refer me to a site or vendor for the cylinder? Thanks, Michael Wright PS No perpetual motion ideas here. Just a study in pneumatics.