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seconds, i.e. a somewhat larger time. As a consequence of its

motion the clock goes more slowly than when at rest. Here also

the velocity c plays the part of an unattainable limiting

velocity.

XIII.

THEOREM OF THE ADDITION OF VELOCITIES. THE EXPERIMENT OF FIZEAU

Now in practice we can move clocks and measuring-rods only with

velocities that are small compared with the velocity of light;

hence we shall hardly be able to compare the results of the

previous section directly with the reality. But, on the other

hand, these results must strike you as being very singular, and

for that reason I shall now draw another conclusion from the

theory, one which can easily be derived from the foregoing

considerations, and which has been most elegantly confirmed by

experiment.