, making use of the first and fourth equations of the Lorentz transformation. Instead of the equation (A) we then obtain the equation
which corresponds to the theorem of addition for velocities in one direction according to the theory of relativity. The question now arises as to which of these two theorems is the better in accord with experience. On this point we are enlightened by a most important experiment which the brilliant physicist Fizeau performed more than half a century ago, and which has been repeated since then by some of the best experimental physicists, so that there can be no doubt about its result. The experiment is concerned with the following question. Light travels in a motionless liquid with a particular velocity
. How quickly does it travel in the direction of the arrow in the tube
(see the accompanying diagram, Fig. 3) when the liquid above mentioned is flowing through the tube with a velocity
?