302 



DISCOVERY 



of our own ; and his later work indicated that his 

 conchision had been correct. It is obvious that 

 only the heavier gases can be components of such an 

 atmosphere ; the lighter gases must have long since 

 escaped into space. Indeed, Pickering declares that 

 " any gas that was not constantly renewed from the 

 moon's interior would have practically disappeared 

 from its surface long ago. Let us now see what gases 

 are at the present time being given off from the earth's 

 interior. We find that there are onlj' two that escape 

 in large quantities — carbonic acid and water-vapour. 

 The former would remain for some time on the moon's 

 surface on account of its weight, and the latter because, 

 on account of the low pressure, the rapid evaporation 

 would cause it immediately to freeze." 



These theoretical considerations form the back- 

 ground of Pickering's theory of the lunar surface, 

 where he believes volcanic activity to persist on a 

 small scale, and hoar-frost to be extensively deposited. 

 Those who accepted the evidence of Schmidt, Klein, 

 and others as to changes on the moon usually at- 

 tributed them to the crumbling of rocks due to the 

 extreme range of temperature. Few believed in the 

 existence even of feebly active volcanoes. Accordingly, 

 when Pickering first saw the small crater at the 

 source of a deep winding cleft known as " Schroter's 

 Valley," he was amazed by its strong resemblance 

 to the crater of a terrestrial volcano in active eruption. 

 Clouds of white vapour, of great density, were seen 

 rising from the bottom of the crater and pouring 

 over its south-eastern wall. Observations at Cam- 

 bridge, Mass., and at Mandeville, confirmed these early 

 results, and showed that this activity commenced a 

 day or two after lunar sunrise, increased to a ma.xi- 

 mum, and ceased a few days before sunset. Even the 

 few examples which Pickering cited in 1903 were 

 sufficiently striking to lend strong support to the 

 view that lunar volcanic activity had not altogether 

 ceased. 



As water-vapour cannot liquefy on the moon's 

 surface, owing to the low pressure, it must immedi- 

 ately freeze. If so, therefore, one would naturally 

 expect to find evidences of snow or hoar-frost on the 

 lunar surface. This is exactly what Pickering has 

 found. He has ascertained that a considerable 

 number of craterlets are lined with a white substance, 

 which becomes more and more brilliant as the sun 

 shines upon it. The same substance is to be seen on 

 certain mountain peaks. Many of these features 

 have been detected, because Pickering has not confined 

 his studies of lunar formations, as most selenographers 

 have done, to the periods of lunar sunrise and sunset 

 when they are most clearly defined. He holds, indeed, 

 that " the only time when the moon is interesting to one 

 who is really used to its surface is when changes are 



taking place on it, and these only occur far from the 

 terminator ; that is, during the lunar summer-time." 



Round the small crater Linn6 is a halo of bright 

 material, which becomes visible a day or two after 

 sunrise. Pickering made a series of measurements of 

 this halo, but they were strangely discordant. At 

 first, indeed, the results seemed inexplicable until it 

 occurred to him " to compare the diameters of the area 

 in question with the number of hours that it had been 

 exposed in each case to the sun. The whole matter 

 then became clear. When the white spot first became 

 visible, one and a half daj^s after lunar sunrise, it was 

 five miles in diameter. As the sun rose the spot 

 rapidly diminished in size, until one day after the lunar 

 noon it was only two and a half miles in diameter. 

 From then till one and a half days before sunset, 

 when it disappeared, it steadily increased in size, 

 reaching a diameter of four miles. During the lunar 

 night it must have continued to increase, until after 

 sunrise it became, as before, five miles in diameter." 



The visibility of snow or hoar-frost on the moon is 

 conditioned by two opposing factors : (i) The snow 

 tends to melt under the direct rays of the sun ; and 

 (2) the snow only becomes visible when illuminated by 

 the sunHght. As the sun rises higher in the sky, it 

 shines into crevices on the surface and iUuminates 

 white regions previously invisible. A striking in- 

 stance of increasing visibility is that of the bright 

 streaks or " rays " surrounding certain prominent 

 craters, more particularly the conspicuous formation 

 known as Tycho. These streaks, which are seen only 

 about the period of full moon, were a standing puzzle 

 to the older selenographers. Nasmyth supposed them 

 to be due to cracks in the surface filled by an outflow 

 of white material from the interior of the moon, jmd 

 in this he was followed by Proctor. Pickering's ex- 

 planation is much simpler. The snow which gives 

 rise to these " rays " lies in crevices, and cannot become 

 visible until the sun has attained a certain altitude. 

 Less convincing, perhaps, is Pickering's explanation of 

 why snow is deposited in these crevices — by winds 

 blowing in certain directions. That the tenuous at- 

 mosphere must have some mode of circulation is mani- 

 fest, however, from recent investigations in 1917 of the 

 minute craterlets near the formation known as Theo- 

 philus. The water-vapour expelled from the crater 

 cone obviously does not freeze as it ascends. There is 

 some form of atmospheric diffusion. " These changing 

 snow areas seem necessarily to involve the existence of 

 low cloud or fog in some form or another, and thin 

 clouds doubtless account for some of the less bril- 

 liant areas observed." Confirmatory evidence of the 

 existence of fog was obtained four years ago by 

 the Italian astronomer Maggini at Florence. On 

 October 10, 1916, while observing Plato, he noted that 



