Measurement of the Velocity of Light in a Partial Vacuum

The observations were made by the rotating-mirror method, the light passing through a steeltube i mile long, evacuated to pressures which ranged from o.5 to 5.5 mm mercury. By multiple reflections the path length was increased to 8 or io miles. The distance was obtained by reference to a carefully measured base line adjoining the tube. The time was measured stroboscopically through successive steps by use of a tuning fork synchronized with the rotating mirror, a free swinging pendulum, a chronometer, and wireless signals from Arlington. There were made 2885.5 determinations of the velocity, the simple mean value of which is 299,774 km/sec., with an average deviation of ii km/sec. from the mean