Ossian Ringbom patented his design of hot air engine in 1907, just as commercial interest in them waned, so his idea was largely lost until Jim Senft started looking at it in the 1980s. The advantage of Ringbom’s patent was that, in his engine, the displacer piston is “free” so his design has less moving parts than any other; the disadvantage, at least initially, was that as Ringbom envisaged gravity being responsible for one half of the displacer’s movement, the engine was likely to be slow. The author realised a way round this releasing the potential of the basic idea, which can be applied to any configuration of hot air engine. Here he briefly describes the idea in straightforward terms and then gives plans and construction details for 3 Ringbom engines, Tapper, a 2cc spirit fired power house, L-27, and LTD Ringbom and Thumper, a beta Ringbom for you to build. To end there is a fascinating chapter on “New Engine Ideas” to really stir up the grey matter. Guaranteed to keep you happy in the workshop! 114 page, well illustrated paperback. Moriya Press