102 
MR. AIRY ON THE ATTRACTION OF MOUNTAINS. 
my present explanation. However, in order to present my ideas in the clearest form, 
I will suppose the interior to be perfectly fluid. Fig. i. 
In the accompanying diagram, fig. 1, suppose 
the outer circle, as far as it is complete, to re- 
present the spheroidal surface of the earth, con- 
ceived to be free from basins or mountains except 
in one place ; and suppose the prominence in the 
upper part to represent a table-land, 100 miles 
broad in its smaller horizontal dimension, and 
two miles high. And suppose the inner circle to 
represent the concentric spheroidal inner surface 
of the earth’s crust, that inner spheroid being 
filled with a fluid of greater density than the 
crust, which, to avoid circumlocution, I will call 
lava. To fix our ideas, suppose the thickness of the crust to be ten miles through 
the greater part of the circumference, and therefore twelve mil6s at the place of the 
table-land. 
Now I say, that this state of things is impossible; the weight of the table-land would 
break the crust through its whole depth from the top of the table-land to the surface 
of the lava, and either the whole or only the middle part would sink into the lava. 
In order to prove this, conceive the rocks to be separated by vertical fissures at 
the places represented by the dotted lines ; conceive the fissures to be opened as they 
would be by a sinking of the middle of the mass, the two halves turning upon their 
lower points of connexion with the rest of the crust, as on hinges; and investigate the 
measure of the force of cohesion at the fissures, which is necessary to prevent the 
middle from sinking. Let C be the measure of cohesion ; C being the height, in 
miles, of a column of rock which the cohesion would support. The weight which 
tends to force either half of the table-land downwards, is the weight of that part of 
it which is above the general level, or is represented by 50 x 2. Its momentum is 
50x2x25 = 2500. The momenta of the “ couples,” produced at the two extremities 
of one half, by the cohesions of the opening surfaces and the corresponding thrusts of 
the angular points which remain in contact, are respectively C X 12 X 6 and C X 10 X 5 ; 
their sum is Cxl22. Equating this with the former, C=20 nearly; that is, the 
cohesion must be such as would support a hanging column of rock twenty miles 
long. I need not say that there is no such thing in nature. 
If, instead of supposing the crust ten miles thick, we had supposed it 100 miles 
thick, the necessary value for cohesion would have been reduced to ^th of a mile nearly. 
This small value would have been as fatal to the supposition as the other. Every 
rock has mechanical clefts through it, or has mineralogical veins less closely con- 
nected with it than its particles are among themselves; and these render the 
cohesion of the firmest rock, when considered in reference to large masses, absolutely 
