They were a little oscillating steam engine used by Darnell in the earlier days of the hobby. Rigged to an electric motor, they can move air about, but they tended to be rather leaky without modifications. Most replaced them with a small compressor culled from the small 12v tyre inflator units you can get in Halfords etc.
Nowadays there are excellent small diaphragm pumps available secondhand on ebay for a few pounds that work much better provided you don't want to pump higher pressures (e.g. over 5-10psi).
The main snag with compressed air is that it can be harder to control for fine trim, so most systems using it tend to have it as a binary system- full or empty, and use an additional trim tank for fine trimming that last 1% of variance owing to water conditions e.g. temperature and mineral content.
Where a single tank system is desired many now use water pumps as they allow fine control and are easier to check.