According to the theories prevailing at the time light traveling through a moving medium would be dragged along by the medium so that the measured speed of the light would be a simple sum of its speed through the medium plus the speed of the medium. In the 1850s French physicist Jean Foucault measured the speed of light in a laboratory using a light source a rapidly rotating mirror and a stationary mirror. His calculation was very close to the current accepted value of the speed of light in a vacuum of 186282 miles per second.
In 1849 Fizeau calculated a value for the speed of light more precise than the previous value determined by Ole Romer in 1676.
Still an inch or two behind. Fizeaus measurement is based on the following idea. Fizeau had previously read about Italian physicist Galileo Galilei who in 1638 predicted that light had a speed that could be measured. Knowing the rotational speed of the cogwheel the width of one cog and the distance to the mirror were all that was required for Fizeau to make the simple calculation of the speed of light.