"What are these streaks? We do not know. During the second quarter of the moon, when the sun is rising slowly upon Tycho, lighting up his peak and showing the crater beautifully divided into a bright cup in the curve to the right, while a dense shadow lies in the left hollow, these streaks are only faint, and among the many craters around (see Fig. 4) you might even have some difficulty at first in finding the well-known giant. But as the sun rises higher and higher they begin to appear, and go on increasing in brightness till they shine with that wonderfully silvery light you see now in the full moon."
Plan of the Peak of Teneriffe, showing how it resembles a lunar crater. (A. Geikie.)]
"Here is a problem for you young astronomers to solve, as we learn more and more how to use the telescope with all its new appliances."
The crater itself is not so difficult to explain, for we have many like it on our earth, only not nearly so large. In fact, we might almost say that our earthly volcanoes differ from those in the moon only by their smaller size and by forming _mountains_ with the crater or cup on the top; while the lunar craters lie flat on the surface of the moon, the hollow of the cup forming a depression below it. The peak of Teneriffe (Fig. 5), which is a dormant volcano, is a good copy in miniature on our earth of many craters on the moon. The large plain surrounded by a high rocky wall, broken in places by lava streams, the smaller craters nestling in the cup, and the high peak or central crater rising up far above the others, are so like what we see on the moon that we cannot doubt that the same causes have been at work in both cases, even though the space enclosed in the rocky wall of Teneriffe measures only eight miles across, while that of Tycho measures fifty-four.
"But of the streaks we have no satisfactory explanation. They pass alike over plain and valley and mountain, cutting even across other craters without swerving from their course. The astronomer Nasmyth thought they were the remains of cracks made when the volcanoes were active, and filled with molten lava from below, as water oozes up through ice-cracks on a pond. But this explanation is not quite satisfactory, for the lava, forcing its way through, would cool in ridges which ought to cast a shadow in sunlight. These streaks, however, not only cast no shadow, as you can see at the full moon but when the sun shines sideways upon them in the new or waning moon they disappear as we have seen altogether. Thus the streaks, so brilliant at full moon in Tycho, Copernicus, Kepler, and Aristarchus, remain a puzzle to astronomers still."