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144 same time by different people may be very different. Fig. 37 shows two drawings made by Mr. Percival Lowell and his assistant on August 14, showing about what they could see at that time; and you see that although they have tried as faithfully as they could to put down what they saw, Mr. Lowell saw a straight canal nearly due North and South, while his assistant saw it slope to the East. When there is difference of opinion of this kind the only way is to make vast numbers of drawings, throwing away what only one man sees, and keeping what everybody sees;

in this way only can they really make a trustworthy chart of Mars. You may say: Why not photograph the planet? But photographing is even less satisfactory because the camera cannot always seize good moments as the eye can; it often photographs at the wrong moment. By taking many photographs, however, one after another, we can pick out the good ones; this plan has only recently been introduced, but it has given us already some good photographs of Mars. Fig. 38 shows three taken by Professor Barnard on which you see the polar caps clearly. But, after all, we shall much better understand the