Page:Mars - Lowell.djvu/86

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the surface than their number would indicate, for the depressions extend as a rule much further both in latitude and longitude.

Usually the depressions look like parings from the planet’s rind, and almost always appear upon that part of the terminator where the dark regions are passing out of sight; commonly therefore, in the case of the southern hemisphere, they are met with between latitudes 30° to 60° south. Not so common is it for them to occur over a part of the planet which is bright. Furthermore, they appear to occur more or less continuously. This would not be the case were they real depressions.

As this may not at once be evident to the reader, and yet is easily made evident, we will consider the diagram on page 38. It will there be seen that an elevation like s or r—and the same reasoning applies mutatis mutandis to a depression—appears projected a relatively long way without or within the terminator, as compared with its actual length, owing to the angles under which it is respectively illuminated by the Sun and seen from the Earth. The relation between its height and its distance from the edge is that between the height of a hill and the shadow it casts at sunrise or sunset. What, therefore, is not high enough to be seen in profile on the limb, becomes vicariously visible on the terminator. But a hill could not continue