170 



KNOWLEDGE 



[Septembkr 1, 1893. 



crosses 

 end of 

 thians ; 

 ranges, 



of some of these rays. If they correspond to lava streams, 

 it will be evident that the lava must have been in a very 

 liquid condition when they ran over the plain ; but they 

 do not seem to have run into valleys forming lakes of lava, 

 or to have spread out at their ends like terrestrial lava 

 streams when they reach a level area. 



Some of the streaks appear to extend across the ridge 

 ■which stretches 

 south ward from Era- 

 tosthenes (see Fig. 

 4). It may be argued 

 that this ridge may 

 have been raised 

 after the epoch when 

 the lava flowed down 

 from the crater 

 of Copernicus. But 

 a similar assump- 

 tion can hardly be 

 made with regard to 

 the two great rays, 

 one of which 

 crosses the Apeu- 

 nine range close to 

 Eratosthenes, and 

 the other of which 

 the western 



the Carpa- 



for if these 



which form 

 the border of the 

 Mare Imbrium, had 

 been raised after the 

 era of the lava flows 

 from Copernicus, 

 the lower ends of 

 the lava streams, 

 where they run over 

 the plain, would 

 have been obliter- 

 ated by the molten 

 sea which levelled 

 the Mare Imbrium. 

 These long rays 

 cannot be much 

 raised above the level 

 of the surrounding 

 surface, for they do 

 not throw any re- 

 cognizable shadows 

 while the sun is 

 rising. They have 

 hazy, nebulous 

 edges, and both sides 

 (that towards the 

 sun and that away 

 from the sun) seem 

 equally soft and inde- 

 finite ; on the other 

 hand, the smaller 



forking streams Fio. 6.— From a photogi-aph taken by 



which appear to have run down from the south side of the 

 crater throw distinct shadows. One is therefore led to con- 

 clude that the long white rays stretching from Copernicus, 

 like those which radiate from Tycho, are merely surface 

 markings, and that they probably follow the lines of volcanic 

 fissures, the whitish colour on either side being produced 

 by some exhalation which has issued from the fissure, and 

 has been precipitated on the surface near to the vent. 



It will be noticed ou comparing Plates I. and II. that 

 the central regions of these rays are the brightest, and that 

 they decrease Ln density or whiteness on either side, as 

 might be expected if vapour issued from a narrow vent 

 running along their centre. Thus, if water vapour issued 

 from the vent, it might, on reaching the cold surface, be 

 deposited as snow or hoar-frost on either side, the amount of 



the deposit being 

 greatest close to the 

 vent. In connection 

 with this assump- 

 tion it should be 

 borne in mind that, 

 owing to the small 

 intensity of lunar 

 gravity, evaporation 

 must take place more 

 energetically for any 

 given change of 

 temperature at the 

 moon's surface than 

 at the earth's sur- 

 face. 



Thus, from labor- 

 atory experiments it 

 is known that the 

 elastic force of water 

 vapour at a temper- 

 ature of— 32° Cent., 

 is such that it will 

 raise the mercury 

 in a barometer 

 through about a 

 third of a millimetre 

 (0-32 mm.), while 

 at a temperature of 

 0° Cent, it raises 

 the mercury through 

 more than 4i milli- 

 metres (4-6 mm.). 

 This increase of 

 pressure is indepen- 

 dent of the pressure 

 of the atmosphere 

 or other gases. It 

 cannot reasonably be 

 doubted that water 

 vapour would be- 

 have in precisely the 

 same way upon the 

 moon as it does 

 here, but in order 

 to raise the pres- 

 sure of the lunar 

 atmosphere by any 

 required amount, 

 about six times as 

 much water vapour 

 would need to be 

 evaporated into the 



the Jirothers Henry on 2Sth May, 1890. lunar atmosphere as 



I would need to be evaporated into the earth's atmos- 

 phere, where gravity is six times as great. The 

 pressure of water vapour is a physical quantity depen- 

 dent on the energy of the molecules, and it could be 

 made to depress or raise a spring balance. But the 

 depression of a spring balance which would raise a 

 column of mercury 4 millimetres on the earth would 

 raise a corresponding column about 24 millimetres ou the 



