338 FROM NEBULA TO NEBULA 



occurs. In the case of Linn 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 is 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 

 to the creation of the larger craters, since we find numerous of 

 these craters partially ruined and dissolved in them, evidences of 

 fusion being everywhere visible. The seas really owe their ex- 

 istence without doubt to extensive fissure eruptions, such as have 

 occurred in past ages in India and in some of our North-western 

 states. During this period extensive areas of the Moon's crust 

 sank beneath the liquid surface and were dissolved in it, much 

 as often occurs at the present time on a small scale in Kilauea, 

 Hawaii. The bottoms of the great majority of the larger craters 

 were also dissolved or softened at this time, indicating that the 



