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It is a different matter when the distance has to be judged from

the railway line. Here the following method suggests itself. If

we call A′ and B′ the two points on the train whose distance

apart is required, then both of these points are moving with the

velocity v along the embankment. In the first place we require to

determine the points A and B of the embankment which are just

being passed by the two points A′ and B′ at a particular time

t-judged from the embankment. These points A and B of the

embankment can be determined by applying the definition of time

given in Section VIII. The distance between these points A and B

is then measured by repeated application of the measuring-rod

along the embankment.