OF THE SURFACE OF THE MOON. 
337 
the clinometer. When, on the surface of the earth, we observe incoherent materials 
resting at a slope approaching 45°, we frequently remark some peculiarity of the sepa- 
rable masses which allows of mutual support, or observe that the usual atmospheric 
agencies, the ordinary changes of temperature and moisture, have had only a very short 
influence on a newly exposed surface. After a very long exposure, even hard and con- 
tinuous rocks, such as those of Malvern, sink down to a slope of 18°; and it is chiefly 
under freshly wasting escarpments of softer strata that we see, as in Mam Tor and 
the west front of Ingleborough, anything approaching to the steepness of the moon- 
crater walls. So in the old volcanic regions of Auvergne and Mont Dor, the Eifel 
and even the Phlegrsean fields, which may be the fittest for comparison with the crater- 
covered tracts of the moon, very steep slopes are almost unknown, except for small 
spaces of consolidated rocks, such as in the Puy de Dome, Monte Somma, and the 
Drachenfels. 
Perhaps we may refer the obviously greater steepness of some of the surfaces on the 
moon to the lesser influence of gravity on the satellite than on the earth. If in a given 
mass of matter, exposed to various causes by which cohesion 
is influenced, we represent by C (vertical) the residue of co- j 
hesive force, and by G (horizontal) the force of gravity, the J 
C I 
slope from C to G will be the final surface, and ^ = tangent I 
of its inclination to the horizon. On the earth, the force of & 
gravity being more than six times as great as on the moon, the tangent of inclination 
will be relatively less. 
Also, if the degrading influences of all kinds on the moon be taken as very much less 
than on the earth, this again will relatively increase the angle of slope ; so that we 
appear to have no difficulty in understanding why lunar mountains may be so much 
steeper and less worn than the objects most like them on earth. Perhaps the same 
general consideration of the greater relative value of heat and other molecular forces, 
when compared with gravity, on the moon than on the earth, may account for the much 
greater amplitude, and the generally greater elevation of the moon mountains ; for it 
is only exceptionally that Chimborazo, Teneriffe, and Elburz ascend on the earth to so 
great a height as the loftiest lunar summits*. 
On the very crest of a ring-mountain it is rare to find a cup-crater; quite common to 
find them in the interior, especially towards the middle, and, in several cases, exactly in 
the centre. But it happens often that the central mountain-mass of a large crater, such 
as Gassendi and Theophilus, is of a different structure. In the former a complicated digi- 
tated mass of elevated land appeared to me for a long time to be entirely devoid of any 
small craters ; by continued scrutiny at last I see on one of the masses a distinct depres- 
sion. The area in Gassendi reminded me of the volcanic region of Auvergne, in which, 
* On the general relation of the moon’s surface to volcanic tracts on the earth, see a graphic passage in 
Herschel’s £ Introduction to Astronomy,’ p. 229, Edition 1, 1833. 
3 A 2 
