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we must then express x′ and t′ in terms of x and t, making use of

the first and fourth equations of the Lorentz transformation.

Instead of the equation (A) we then obtain the equation

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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 w. How quickly does

it travel in the direction of the arrow in the tube T (see the

accompanying diagram, Fig. 3) when the liquid above mentioned is

flowing through the tube with a velocity v?

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