PHOTOGRAPHY OP THE LUNAR SURFACE. 117 



less completely, according to the season, filling the cavities which there 

 abound or circulating in capillary fissures, compressing and expelling 

 any gases which are encountered. The same phenomenon must also 

 be going on at the sea bottom, and in addition gravity always tends to 

 remove the water farther away from the surface. The absorption is, 

 however, limited by the internal heat of the globe. At a depth of 3,000 

 or 4,000 meters the water would be brought in contact with layers at 

 a temperature of 100°. It would therefore be vaporized and the ten- 

 sion of the vapor formed would be sufficient to partly drive back the 

 water into the upper layers or even to spurt it out in the form of 

 thermal springs. This subterranean circulation of the water is prob- 

 ably the most active cause of the loss of the internal heat of the globe. 

 In proportion as the cooling progresses the depth of the layer, in which 

 water can exist in the liquid state, would increase and its capacity for 

 absorption would thus be still farther augmented. 



If we give due weight to the difference in the physical conditions on 

 the earth and on the moon we become, without difficulty, convinced 

 that all the causes which have tended to diminish the height of the 

 atmosphere and the volnme of the ocean on our globe must have had a 

 still greater influence on our satellite. Less well provided with air and 

 water originally, and with a greater extent of surface in proportion to 

 its volume, the moon would take less time to consume the reserve sup- 

 plies. The formation of saline compounds, of limestone, gypsum, and 

 nitrates must have been able to effect a nearly complete absorption of 

 the water before even the cooling would have permitted its condensation 

 on the surface. The remainder, having escaped these causes of absorp- 

 tion or having been again set at liberty by volcanic eruptions, must 

 have filtered into the innumerable orifices of every size which honeycomb 

 the lunar core. In addition, the j>enetration would be more easily 

 effected than on the earth, for the internal temperature, decreasing 

 more rapidly with the depth, would not tend to prevent that action to 

 the same extent. To briefly summarize the above it must be consid- 

 ered as established by observation that the moon does not at present 

 contain either bodies of water having a free surface or an appreciable 

 atmosphere. If it be demonstrated, however, that water in the form 

 of vapor has actively contributed to the formation of the present relief, 

 we can not tell a priori whether water has ever circulated in the liquid 

 form at some intermediate period or whether it had time to be con- 

 densed on the surface before being absorbed, before accomplishing 

 important mechanical effects, or before finally giving rise to the forma- 

 tion of great accumulations of ice. 



On these different points there is abundant opportunity for interro- 

 gating unprejudiced observation, and for examining if our satellite 

 shows any evidences of eroded valleys, sedimentary deposits, and ice 

 fields of considerable extent — phenomena which ought to result from 

 the prolonged action of water. Modern photographs will throw much 

 valuable light on this subject. 



