Sronry— Of Atmospheres upon Planets and Satellites. 317 
This is what would happen if the Moon were by itself, and if portions of its 
surface could rise even to a temperature of — 66°C. But the conditions are more 
favourable. Lord Rosse infers from his observations that the temperature of the 
Moon’s surface rises something like 280° C. when exposed to the fierce glare of the 
Sun’s rays. Even if it shall turn out that this is an overestimate, it at all events 
makes it probable that the maximum temperature is very much higher than 207° 
above the absolute zero, which is the same as 66° below the freezing point. More- 
over the proximity of the Earth would somewhat assist the process at its present 
distance ; and its greater proximity in former ages must have more assisted it. 
In fact, on this account, any of the gases or vapours in question which had been 
developed upon the Moon while the Moon was close to the Earth must have been 
for the most part transferred over tothe Karth, if the Earth was then cool enough 
to retain them. Those molecules that have escaped from the Moon since its 
distance from the Earth became considerable have for the most part become 
independent planets travelling in a ring round the Sun, of which ring (roughly 
speaking) the Earth’s path is the central line. There they are accompanied by 
most of the molecules of hydrogen and helium that have leaked away from the 
Earth. <A very few of the latter which happened to be shot off at unusually high 
speed, and in the direction towards which the Earth was at the time travelling in 
its orbit, may have been able to disengage themselves altogether from the Solar 
system ; but this can have happened to but few of those thrown off from the Earth, 
and not to almost any of those ejected from the Moon. 
Cuapter VII.—Of Mercury. 
The radius of Mercury may be obtained by assuming the equatorial radius of 
the Earth to be 6378 kilometres, and applying to it the data given in the preface 
to the Nautical Almanac for 1899. We thus find the planet’s radius 
B34 
? = 7848 6378 = 2406 km. 
The mass of Mercury is less satisfactorily known. We shall use the value 
m 
= = 0:065. 
Wh 0 
Mercury’s rotation period is also in doubt. » The difficult observations that have 
hitherto been made seem to be about equally consistent with a rotation period of 
nearly a day, and a rotation period of 88 days (the period of Mercury’s revolution 
