Poincar\'e's Dynamics of the Electron - A Theory of Relativity?
  Before 1905, Poincar\'e stressed the importance of the method of clocks and their synchronization, but unlike Einstein, magnet and conductor (asymmetries in Lorentz's theory regarding the explanation of Faraday's induction) or chasing a light beam and overtaking it, were not a matter of great concern for him. In 1905 Poincar\'e elaborated Lorentz's electron theory from 1904 in two papers entitled "Sur la dynamique de l'electron". In May 1905 he sent three letters to Lorentz at the same time that Albert Einstein wrote his famous May 1905 letter to Conrad Habicht: "I can promise you in return four works,[...] The fourth paper is only a rough draft at this point, and is an electrodynamics of moving bodies". In the May 1905 letters to Lorentz Poincar\'e presented the basic equations of his 1905 Dynamics of the Electron. Hence, in May 1905, Poincar\'e and Einstein both had drafts of papers pertaining to the principle of relativity. Poincar\'e's draft led to a space-time mathematical theory of groups at the basis of which stood the postulate of relativity, and Einstein's draft led to a kinematical theory of relativity. Poincar\'e did not renounce the ether. He wrote a new law of addition of velocities, but he did not abandon the tacit assumptions made about the nature of time, simultaneity, and space measurements implicit in Newtonian kinematics. Although he questioned absolute time and absolute simultaneity, he did not make new kinematical tacit assumptions about space and time. He also did not require reciprocity of the appearances, and therefore did not discover relativity of simultaneity: these are the main hallmarks of Einstein's special theory of relativity. Nevertheless, as shown by other writers, Poincar\'e's theory had influenced later scientists especially Hermann Minkowski. 
Galina Weinstein
physics.hist-ph