Author: Donal Deering CEng MIEI Dear Editor, Gerard Palmer's recent letter on using gravity as a source of renewable energy got my attention. I can see nothing wrong in his analysis and he acknowledges that there would be an need for an energy input to keep such a device in motion. Thus, his idea is different from those that promise perpetual motion with no energy input. These latter devices are generally discredited and contradict all our well-known laws of physics and thermodynamics. The challenge, I think, will be to design such a device and to still get a meaningful output. Theoretically, one could get 500W using Gerard's figures, assuming a 50kg mass, a height of 2m and a cycle time of two seconds. Then, of course, there will be losses and the real output will be considerably less. There may not even be any positive result. Anyway, its an interesting challenge and I look forward to further discussion here. Donal Deering CEng MIEI, Kilkenny