376 FROM NEBULA TO NEBULA 



The point of first interest perhaps in this investigation is to 

 find when the spots reach their minimum size. Within the 

 craters, the dark areas appear and disappear at about the same 

 interval before and after midday, which therefore seems to be 

 the time when the snow presents the smallest area. In the case 

 of Linne, Professor Barnard and the writer agreed that the mini- 

 mum occurred one terrestrial day after the lunar midday. * * * It 

 would certainly be of interest to prepare a series of drawings of 

 the craterlet near Littrow B, and determine when its minimum 

 occurs. In the case of Linne and apparently also of Littrow, the 

 white spot is invisible both at sunrise and sunset. Just why this 

 should be so is not very clear, but it would seem to indicate that 

 the moisture can only escape from the vent about midday, and 

 that towards sunset it all evaporates. Towards noon the evap- 

 oration occurs before it can get far from the vent, hence the 

 spot is smaller, although brighter at that time than earlier or 

 later. * * * 



In the case of the mountains, Pico, Pico B, and Straight 

 Range, most of the white spots grow smaller the longer the sun 

 shines on them. Those on the west side of the mountains, to- 

 wards the rising sun, are of full brightness when the sun first 

 strikes them. Those on the east do not deposit until the sun has 

 been shining on the region for a day or two. It appears as if the 

 ground some little way beneath the surface must be heated up be- 

 fore the moisture can escape. * * * 



The writer has sometimes been asked, "What reason is there 

 to believe that there is ice upon the moon?" The answer is: 

 "For the same reason that we believe there is ice upon Mars, 

 because the phenomena observed can be more readily explained 

 that way than any other." Whether the ice is deposited upon 

 the surface, or floats as minute crystals just above it, in the form 

 of surface clouds or fog, is not yet clear, but it is believed it oc- 

 curs in both forms. Where the boundaries are sharply defined, 

 it lies upon the surface. Where the boundaries are indistinct and 

 hazy, as for instance in the case of Linne, it is still uncertain. In 

 the case of the bright rays surrounding Tycho, it is thought the 

 ice crystals are supported in the lunar atmosphere like those 

 terrestrial cirrus clouds to which we give the name of mare's 

 tails. * * * 



The question whether water formerly existed upon the 

 moon in larger quantities than at present is usually answered in 

 the affirmative, but generally for wholly erroneous reasons. We 

 often hear it stated that the so-called seas are simply old ocean 

 bottoms, a most improbable view. A casual examination of the 

 lunar surface shows that they were formed at a period subsequent 



