MR. AIRY ON THE ATTRACTION OF MOUNTAINS. 
103 
insignificant. The miners in Cornwall know well the danger of a “fall” of the firmest 
granite or killas^ where it is undercut by working a lode at an inclination of 30° or 
40° to the vertical. 
We must therefore give up the supposition that the state of things below a table- 
land of any great magnitude can be represented by such a diagram as fig. 1. And 
we may now inquire what the state of things really must be. 
The impossibility of the existence of the state represented in fig. 1 has arisen from 
the want of a sufficient support of the table-land from below. Yet the table-land 
does exist in its elevation, and therefore it is supported from below. What can the 
nature of its support be ? 
I conceive that there can be no other support than that arising from the downward 
projection of a portion of the earth’s light crust into the dense lava ; the horizontal 
extent of that projection corresponding rudely with the horizontal extent of the table- 
land, and the depth of its projection downwards being such that the increased power 
of floatation thus gained is roughly equal to the increase of weight above from the pro- 
minence of the table-land. It appears to me that the state of the earth’s crust lying 
upon the lava may be compared with perfect correctness to the state of a raft of tim- 
ber floating upon water ; in which, if we remark one log whose upper surface floats 
much higher than the upper surfaces of the others, we are certain that its lower sur- 
face lies deeper in the water than the lower surfaces of the others. 
This state of things then will be represented by fig. 2. Adopting this as the 
true representation of the arrangement of masses Fig. 2. 
beneath a table-land, let us consider what will 
be its effect in disturbing the direction of gra- 
vity at different points in its proximity. It will 
be remarked that the disturbance depends on 
two actions ; the positive attraction produced by 
the elevated table-land ; and the diminution of 
attraction, or negative attraction, produced by the 
substitution of a certain volume of light crust (in 
the lower projection) for heavy lava. 
The diminution of attractive matter below, pro- 
dueed by the substitution of light crust for heavy 
lava, will be sensibly equal to the increase of at- 
tractive matter above. The difference of the negative attraction of one and the 
positive attraetion of the other, as estimated in the direction of a line perpendicular 
to that joining the centres of attraction of the two masses (or as estimated in a hori- 
zontal line), will be proportional to the difference of the inverse cubes of the distances 
of the attracted point from the two masses. 
* A “fall” occurred in the Dolcoath mine, while I was engaged there with Messrs. Whewell, Sheep- 
shanks, and other friends, on pendulum- experiments, in 1828. 
