factors at work. By taking the average of the series of observed values according to some definite principle, not only do we eliminate a very large class of errors, but we allow by so doing the various causes to unmask their separate results. The importance of reasoning upon averages could hardly be more strikingly exemplified than in the very case before us,—that of these depressions and projections seen on the terminator of Mars.
Of the 694 irregularities measured, 291 were projections and 403 were depressions. Here at the very outset, then, we perceive an objection to the theory that they are due to mountains; to wit, because the number of depressions so greatly exceeds the number of projections. As previously explained on page 64, mountains would produce on the average as many project ions as depressions, for they would project the light on the one side as much as they would cut it off on the other.
Now let us classify these irregularities, and see if we can gain further information about them. There were two kinds of them,—the long and low, and the short and sharp. Each kind had its representatives among both the projections and the depressions. Of the short and sharp variety there were 95 projections. These averaged 0.276 seconds of arc in height. Of the same kind there were similarly 57 depres-
