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Bill Mollison explains a Trompe:

Beginning with a small stream, we can let it out, leading it towards a box tunnel into which the water can free fall. At the top of this tunnel, we can install a large funnel apparatus with holes drilled into it and pipes, like drinking straws, leading to the holes and creating tiny air bubbles. As the water falls through the funnel, it pulls air with it into the tunnel, and because the water is falling faster than the tiny air bubbles, the air becomes trapped below. We can design a large underground chamber into which the air bubbles collect and become highly compressed with the water moving on unused. This air is isothermically compressed.

Isothermically compressed air is tremendously useful. It’s very clean, free of the vaporized oil found in air out of compressed, which means it could be used for scuba diving. More significantly, though, a little pipe could be run to the chamber, and isothermically compressed air can be attached to a shop with a spigot to give access to it. That air could then be used to drive any machine, i.e. power tools. It could be led into an insulated room and released as refrigeration and freezing. It could be bottled and used to run motors, actual cars, with the exhaust being cold rather than hot.