514 THE POPULAR SCIENCE MONTHLY. 



moon, when no light falls within their silent hollows ! What reser- 

 voirs 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 open- 

 ings to the brim, and extending all the length of the cracks. 



You have already noticed that besides the craters there are innu- 

 merable 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 cen- 

 tral 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 evapo- 

 rating oceans ? 



The longer one studies this lonely globe, so beautiful in its desola- 

 tion, 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 repre- 



