Author: Gerard Palmer CEng MIEI Dear Editor, I found myself pondering the other day about renewable energy and I wondered if we are missing a trick by considering only wind, water and wave as energy sources. What, I thought, about gravity? It is readily available and free. Supposing a heavy piston (weight w) were released at the top of a vertical tube (height h) and allowed to fall under the influence of gravity and, at the bottom of the tube were strong springs such that the piston rebounded to, say, half the height. With a small auxiliary motor to lift the piston to the top of the tube, the cycle could be repeated. Input 0.5hw and output 1.5hw. The contraption could be made in whatever size was suitable or, indeed, with multiple units coupled together. A sort of mechanical equivalent of the air or ground-source heat pump. Perhaps readers better qualified than I might be able to say if this is a feasible proposition, or the ramblings of an old man’s mind rapidly going to seed?! Yours faithfully, Gerard Palmer CEng MIEI Dundrum,Dublin 14.