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154 purely natural causes. Indeed, such is the first impression upon getting a good view of them. How instant this inference is becomes patent from the way in which drawings of the canals are received by incredulously disposed persons. The straightness of the lines is unhesitatingly attributed to the draughtsman. Now this is a very telling point. For it is a case of the double-edged sword. Accusation of design, if it prove not to be due to the draughtsman, devolves ipso facto upon the canals.

We have thus far considered the aspect of the canals viewed at any one time. We have now to consider an even more interesting branch of the subject, their consecutive appearances. The "open sesame" to our comprehension of the physical condition of Mars lies in systematic study of the appearances the planet's surface presents night after night and month after month. For that surface changes; and the order, extent, and character of its changes contain the key to their explanation. True as this is of the larger markings upon the disk, it is if anything more noticeably the case with the finer detail of the canals.

After the fundamental fact that such curious phenomena as the canals are visible, is the scarcely less curious one that they are not