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. I^ow^ if there be two spheres, 

 of the same average density, the attraction of either for a 

 point on its surface will 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 ^^^^-^ viz., it Avill vary altogether directly 

 as the radius of the attracting sphere. 



Now, according to Humboldt, the diameter of the moon is 

 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 

 Avill be found from the following compound proportion : — 

 1816 : 6864 : : 1 

 •619 : 1000 



The result of wdiich is 6-1 for the value of terrestrial gravi- 

 tation, or upwards of six times that of lunar gravitation. We 

 need not therefore be surj)rised at matter remaining stable on 

 the moon's surface, in a position from Avhich 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 eacli 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 Avould not lie so closely to the surfiice 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. 



