..."Warming Engine". Click to enlarge the images.
Kelvin's "Warming Engine", whilst it undoubtedly consumed far more energy than it produced, proved a vital principle in thermodynamics which is too often overlooked today.
It was the first Heat Pump.
How it works:
When a gas is rarefied its pressure drops, and also its temperature. In Kelvin's device a large-diameter piston, in a brick-lined cylinder, is withdrawn by motive force. The rarefied air in the cylinder drops in temperature, and heat flows through the walls of the cylinder until an equilibrium is reached. The air in the cylinder is now at ambient temperature, but still at reduced pressure.
The piston is then permitted to return to its primitive position, and the pressure in the cylinder returns to atmospheric. This is effectively a compression, as far as the air is concerned, and its temperature rises, concentrating the heat absorbed through the cylinder walls.
At the end of the piston's return stroke a flap opens and spills the hot air into the house.
See also The Liquid Piston - a project for a low-tech heat pump.