The Lorentz-Larmor ether theory has often been criticized because of its "ad hoc" assumptions about the contraction of bodies and slowing down of clocks moving with respect to the physical vacuum ("ether")*. The object of this paper is to use Poincare's principle about the impossibility to detect motion with respect to the ether ("absolute motion") in order to derive the functions describing the behaviour of moving bodies and clocks. Poincare probably began considering this principle in about 1895  . It has met renewed interest recently, especially in Podlaha's work  . For other recent results in ether theory cf. also Refs.  and  . Besides Poincare's principle we shall assume homogeneity of space and time, isotropy of space, and the existence of a wave in the ether (in the following called "light") travelling at the velocity c in all directions. As we shall see, given these assumptions, a very simple argument suffices to arrive at the conclusion that moving bodies undergo no transversal change of dimension, but a contraction in the direction of motion by the factor ] / l — w2/c2, and moving clocks are slowed down by the same factor.