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and many chapters have been written on this subject. The whole difficulty arises from the different ideas which people have of motion. In physics we say that a body does not rotate when, if a rod were passed through it, that rod always maintained the same direction when the
Fig. 21.—Showing how the Moon would Move if it did not Rotate on its Axis. body moved about. Now let us suppose such a rod passed through the moon; then, if the latter did not rotate on its axis the rod would maintain its same direction while the moon, revolving around the earth, would appear at different points in its orbit as we see it in Figure 21. A very little study of this figure will show that as the moon went around we should successively see every part of its surface in succession if it did not rotate on its axis.

All of us who live on the seashore know that there is a rise and fall of the ocean which in the general average occurs about three quarters of an hour later every day, and which keeps pace with the apparent diurnal motion of the moon. That is to say, if it is high tide to-day when