The Michelson Morley Experiment

If sound worked on the same principles as light, our observer would hear everything just as if he were at rest. This of course is an experiment which cannot be performed, but others which can be performed will show the difference. We might find some place on a railway where there is an echo from a place further along the railway. Say a place where the railway goes into a tunnel. If the train is traveling towards the echo, the passengers will hear it sooner than the man on the bank. But these are not quite the circumstances of the Michelson Morley experiment. The mirrors in that experiment correspond to the echo, and the mirrors are moving with the earth, so that echo ought to move