THE MOON. 



moon a thousand times less dense than our own, its presence must 

 he detected. 



13. The earth's atmosphere supports a column of 30 inches 

 of mercury: an atmosphere 1000 times less dense would support 

 no more than the thirtieth of an inch. It may therefore he con- 

 sidered as proved that the space around the surface of the moon 

 is as exempt from an atmosphere as is the receiver of a good air- 

 pump after that instrument has exhausted it of air to the utmost 

 limit of its power. In fine for all practical purposes it is demons- 

 trated that the moon has no atmosphere. 



14. The same physical tests which show the non-existence of 

 an atmosphere of air upon the moon are equally conclusive against 

 an atmosphere of vapour. It might, therefore, be inferred that 

 no liquids can exist on the moon's surface, since they would be 

 subject to evaporation. Sir John Herschel, however, ingeniously 

 suggests that the non-existence of vapour is not conclusive against 

 evaporation. One hemisphere of the moon being exposed con- 

 tinuously for 328 hours to the glare of sunshine of an intensity 

 greater than a tropical noon, because of the absence of an atmo- 

 sphere and clouds to mitigate it, while the other is for an equal 

 interval exposed to a cold far more rigorous than that which 

 prevails on the summits of the loftiest mountains or in the polar 

 regions, the consequence would be the immediate evaporation of 

 all liquids which might happen to exist on the one hemisphere, 

 and the instantaneous condensation and congelation of the vapour 

 on the other. The vapour would, in short, be no sooner formed 

 on the enlightened hemisphere than it would rush to the vacuum 

 over the dark hemisphere, where it would be instantly condensed 

 and congealed, an effect which Herschel aptly illustrates by the 

 familiar experiment of the CRYOPHOROUS. The consequence, as he 

 observes, of this state of things would be absolute aridity below 

 the vertical sun, constant accretion of hoar frost in the opposite 

 region, and perhaps a narrow zone of running waters at the 

 borders of the enlightened hemisphere. He conjectures that this 

 rapid alternation of evaporation and condensation may to some 

 extent preserve an equilibrium of temperature, and mitigate the 

 severity of both the diurnal and nocturnal conditions of the 

 surface. He admits nevertheless, that such a supposition could 

 only be compatible with the tests of the absence of a transparent 

 atmosphere even of vapour within extremely narrow limits ; and 

 it remains to be seen whether the general physical condition of 

 the lunar surface as disclosed by the telescope be not more com- 

 patible with the supposition of the total absence of all liquid 

 whatever. 



It appears to have escaped the attention of those who assume 

 40 



