XII

As judged from $K$, the clock is moving with the velocity $v$; as judged from this reference-body, the

time which elapses between two strokes of the clock is not one second, but ${\displaystyle \frac{1}{\sqrt{1-{\displaystyle \frac{v^2}{c^2}}}}}$ 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.