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184 his knowledge. The photographs sent to him all had marked on them the date and time when they were taken, but Mr. Saunder was able to say that in one case the wrong month had been given and in another case the wrong day! You can imagine that the astronomer who had taken the photographs did not like being told that he had made mistakes of that kind, and at first he was inclined to dispute it; but Mr. Saunder's case was too strong for him. Now, I want you to realize what this means: it is not as though anything were in a different place on the Moon itself from one day to another; so far as we can judge everything that we can see remains perfectly steady; otherwise Mr. Saunder could not make two different photographs fit. But on different days the Earth looks at the Moon from a slightly different angle, which can be allowed for if you know the correct date. If the wrong date is given, the wrong allowance is made and the measures will not fit those of other photographs. This was what Mr. Saunder found; but I need scarcely say that if his work had not been wonderfully accurate, he could not have found it out; and even as it was, it took him several days' hard work at his calculations, for all such work involves a great deal of arithmetic.

When we make a map of the Earth we may put in the places of the mountains without saying how high they are; but the best maps tell you the heights of all the hills. It is wonderful to think that we can also find the heights of the mountains and hills of the Moon, so that we can make not only a map, but a relief model. The Royal Astronomical Society has kindly lent us a beautiful relief model of a part of