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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 $v$?

In accordance with the principle of relativity we shall certainly have to take for granted that the propagation of light always takes place with the same velocity $w$ with respect to the liquid, whether the latter is in motion with reference to other bodies or not. The velocity of light relative to the liquid and the velocity of the latter relative to the tube are thus known, and we require the velocity of light relative to the tube.

It is clear that we have the problem of Section VI again before us. The tube plays the part of

Fig. 3

the railway embankment or of the co-ordinate system $K$, the liquid plays the part of the carriage or of the co-ordinate system $K'$, and finally, the light plays the part of the man walking along the carriage, or of the moving point in the present