Physical Condition of the Moon's Surface. 93 
‘same as if all the matter of which the sphere is composed 
were collected in its centre. Now, if there be two spheres, 
of the same average density, the attraction of either for a 
point on its surface wili be equal to that produced by the 
whole mass acting from the centre; and, since attraction 
varies inversely as the square of the distance, that on the 
point will vary as “2 viz., it will vary altogether directly 
as the radius of the attracting sphere. 
Now, according to Humboldt, the diameter of the moons 
1816 geographical miles, and the mean diameter of the earth 
is 6864 geographical miles; hence, supposing the density of 
these two bodies to be the same, these numbers will represent 
their proportional attraction for a point on their surface. But 
if the density of our earth be denoted by (1), that of the 
moon, (according to Humboldt), is only °619; if, therefore, 
lunar gravitation be reckoned unity, terrestrial gravitation 
will be found from the following compound proportion :— 
ISIG— 5 6804 ost 
on ee ONG) 
The result of which is 6-1 for the value of -terrestrial gravi- 
tation, or upwards of six times that of lunar gravitation. We 
need not therefore be surprised at matter remaining stable on 
the moon’s surface, in a position from which it would be 
hurled by its own weight if on the earth’s surface. And if 
we suppose that gravitation has exerted a direct influence in 
rounding the irregularities of the earth’s surface, we need 
not be surprised if the moon’s surface be less rounded, and 
more mountainous and irregular. 
In like manner, were each square inch of the moon’s 
surface charged with the same mass of atmosphere as the- 
same extent of the earth’s surface, its tension would be less, 
because its weight would be less. It would therefore be less 
condensed, and would not lie so closely to the surface as on 
our earth, but would spread to a greater distance from it. 
Take a star near the edge of the moon. We may consider 
the star as a luminous point so distant that the rays which 
fall upon the eye form a parallel pencil in passing near the 
moon, and through its atmosphere, if there be any. But the 
mass of material particles which such a pencil will encounter 
in its passage through an atmosphere will vary, not only with 
the total mass of that atmosphere, but also with the manner 
in which the atmosphere is attached to the surface of the 
planet. The following approximate demonstration may serve 
to make my meaning plain. 
