PHOTOGRAPHY OF THE LUNAR SURFACE. 115 



measurements, with sufficient exactness, and consequently we are not 

 able to affirm that the discordance pointed out may not be due to other 

 physical causes. However, it remains established that the principal 

 reason usually given for denying the existence of an appreciable atmos- 

 phere around the moon has actually no value, and the probabilities 

 should rather be considered as in favor of its existence. 



From this ensemble of facts the conclusions may be drawn that the 

 density of the atmosphere on our satellite is certainly very much less 

 than it ought to be, if its division between the moon and the earth had 

 originally taken place in proportion to their masses. This result might, 

 moreover, have been foreseen by theory. Indeed, during the separation 

 of the earth and moon there must have been a considerable time during 

 which the two planets were enveloped in a common atmosphere. The 

 action of masses at a distance being governed by the law of universal 

 attraction, it would necessarily follow that the atmosphere would be 

 divided, not in the ratio of 1 to 8, which is that of the two masses, but 

 in the ratio of 1 to 729, which is that of the volumes of the attracting 

 spheres. 



This remark permits us to consider the small relative density of the 

 lunar atmosphere, verified by observation, as being perfectly in har- 

 mony with Laplace's hypothesis. 



Should one conclude from the above that our satellite has always 

 possessed only an extremely rare atmosphere, incapable of sustaining 

 life, and of being the seat of meteorological phenomena of importance? 

 By no means. It may be supposed that at the epoch when the divi- 

 sion of the atmospheres of the two globes was accomplished, that of 

 the earth was incomparably more extended than to-day. An eleva- 

 tion of the the temperature of a few hundred degrees would transform 

 the water of the ocean into water vapor and would also set free all the 

 carbonic acid locked up in limestone formations. The small portion 

 of the common atmosphere with which our satellite was endowed 

 ought, then, on this principle, to have had a density much greater than 

 its present value. 



The examination of recent photographs furnishes us with numerous 

 and decisive data even on this important, question, enabling us to 

 affirm with certainty that the lunar atmosphere was at some previous 

 time much more dense. This conclusion is almost forced upon us by 

 intense and manifold volcanic phenomena, of which we discover unde- 

 niable traces in nearly every region of the moon. 



Even if the inferior density of the materials displaced be considered, 

 explosions and upheavals could not have been produced on such a 

 large scale without having been accompanied by an abundant disen- 

 gagement of gases. It must be evident, from our knowledge of active 

 volcanoes, that no agent could have supplanted water vapor in this 

 function. The scattering of eruptive products to great distances in 

 the form of radiating streaks which clear all obstacles can not be com- 



