Page:Popular Science Monthly Volume 19.djvu/530

514 moon, when no light falls within their silent hollows! What reservoirs of fiercest heat when, like a giant eye, the sun glares down and floods their unsheltered spaces!

Let us turn once more to those bright fountains and rivers of silver, astronomically called "rilles." Here they are, brilliant as ever; but we can learn nothing of their nature by gazing at them. The astronomer will tell you the latest theory—namely, that when the moon's crust was cooling, ages ago, it contracted, causing immense corrugations, wrinkles, and nodules, and in many places deep rents, admitting water to the heated nucleus, producing volcanic action; in many of these fissures rose up molten matter, filling the central openings to the brim, and extending all the length of the cracks.

You have already noticed that besides the craters there are innumerable craterlets. Your guide will explain again—referring to some great authority—the fanciful and plausible theory that these were the result of downfalls of meteoric rain when the lunar crust was still in the plastic state.

Let us observe the plains again: near the left border, under the sun-glare, they are too brilliant for definition of detail; near the central view their greenish-gray surfaces may be examined in the apparent twilight of the moon. Their seemingly smooth character and color prove them to be beds of oceans of the past lunar ages. These marine bottoms are not smooth in reality, but are seamed and traversed by ranges of hills and mountains, and craters thousands of feet deep! Did these, like monster mouths, swallow the remnants of the evaporating oceans?

The longer one studies this lonely globe, so beautiful in its desolation, the more real does it become to the eye. Here rise veritable mountains casting their black shadows on the plains. There stretch deserts thousands of miles in length, visible throughout all their breadth, for there is little if any perspective on the moon.

To the east is reigning the brilliant lunar day, so long, so fierce, so hot; beyond it is evening, with sunset touching the mountain-peaks on the terminator; in the remotest west, midnight!

This is more than one can see on one's own terrestrial ball, where the vision is bounded by atmosphere, and objects "by degrees grow beautifully less."

One must not look at the full moon to view all these wonders, for seen through the telescope it is merely a brilliant, dazzling sphere; mountains, valleys, and plains are flooded with intensest light; no shadows fall; the white glitter is intolerable!

It must be viewed in six phases: the three-days-old crescent, five days' old, seven days' old or first quarter, the last quarter, and the last three days of the old moon; thus may be seen the four visible sevenths of the lunar globe, all that is ever seen by mortal eye.

From time immemorial the graphic art has been employed in