detect the one to the same extent that we can the other. The value, then, for the Martian twilight arc of 10° is simply a minimal value, not an absolute one. The twilight arc cannot, from the observations, be less than this, but it may be much more.
The large number of measures from which the above means were deduced not only renders error in the result less likely, but shows that result to be due to air pure and simple. This appears from the fact that the observed increase is systematic. For its systematic character proves it due to something largely transparent. It is because it is chiefly not seen that it is seen at all. At first sight this deduction seems paradoxically surprising. But, in considering the problem, we shall realize that it must be so.
If what was seen were opaque, as, for example, a mountain, then in certain positions it would indeed be seen projecting beyond the terminator, — for example, if it were at s in the diagram on page 38; if, on the other hand, it were in the position r, it would, instead of apparently increasing, decrease the diameter. Now, as the rotation of the planet would bring it eventually into all possible positions, it would be as likely on any one occasion to be measured in a position to decrease the diameter as to increase it. From but a few measures, there-
