 In the interest of efficiency, Groves approved an abbreviated process by which Oppenheimer vouched for senior scientists, and three other employees were sufficient to vouch for a junior scientist or technician.The Los Alamos Laboratory was reinforced by a British Mission under James Chadwick.The first to arrive were Otto Frisch and Ernest Titterton; later arrivals included Niels Bohr and his son Aage Bohr, and Sir Geoffrey Taylor, an expert on hydrodynamics who made a major contribution to the understanding of the Rayleigh–Taylor instability.