Page:A Voyage in Space (1913).djvu/205

Rh the Moon's surface, with a model of the neighbourhood of Vesuvius alongside it, so that we can compare the two. You see that on the whole the Earth's mountains are distinctly smaller than those on the Moon. One reason for this is that our mountains are being continually washed away by the rain which falls on them, so that they are smaller now-a-days than they used to be. On the Moon there is no rain, and so the mountains remain unwashed.

One way of measuring the heights of a mountain in the Moon is to measure the length of the shadow it casts. Of course a shadow does not always remain the same length; as you walk past a lamp-post you can see your own shadow grow shorter and longer; but if you stay in the same spot near the lamp it will remain of the same length; or if you go back to that spot it will come back to the same