Poincaré on clocks in motion

Recently-discovered manuscripts throw new light on Poincaré׳s discovery of the Lorentz group, and his ether-based interpretation of the Lorentz transformation. At first, Poincaré postulated longitudinal contraction of bodies in motion with respect to the ether, and ignored time deformation. In April, 1909, he acknowledged time deformation due to translation, obtaining thereby a theory of relativity more compatible with those of Einstein and Minkowski.