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152 their being bigger at one end than at the other. We see examples of what might result from such action in the cracks that radiate from Tycho, in the Moon, or, as we now know from Professor W. H. Pickering's observations, from the craterlets about it. These cracks bear no resemblance whatever to the lines on Mars. They look like cracks; the lines on Mars do not. Indeed, it is safe to say that the Martian lines would never so much as suggest cracks to any one. Lastly, the different radiations fit into one another absolutely, an utter impossibility were they radiating rifts from different centres.

In the same way we may, while we are about it, show that the lines cannot be several other things which they have more or less gratuitously been taken to be. They cannot, for example, be rivers; for rivers could not be so obligingly of the same size at source and mouth, nor would they run from preference on arcs of great circles. To do so, practically invariably, would imply a devotion to pure mathematics not common in rivers. They may, in some few instances, be rectified rivers, which is quite another matter. Glaciation cracks are equally out of the question,—first, for the causes above mentioned touching cracks in general; and, second, because there is, unfortunately, no ice where they occur. Nor can the lines be