104 
MR. AIRY ON THE ATTRACTION OF MOUNTAINS. 
Suppose then that the point Cis at a great distance, where nevertheless the positive 
attraction of the mass A, considered alone, would have produced a very sensible effect 
on the apparent astronomical latitude, as ten seconds. The effect of the negative 
attraction of B will be 10 "X;T 7 r^; and the whole effect will be 10" x — ^ , which 
probably will be quite insensible. 
But suppose that the point D is at a much smaller distance, where the positive 
attraction of the mass A would have produced the effect n". The whole effect, by the 
same formula, will be or n''x ; and as in this case thefrac- 
DA 
tion — is not very nearly equal to 1, there may be a considerable residual disturbing 
DB 
attraction. But even here, and however near to the mountains the station D may be, 
the real disturbing attraction will be less than that found by computing the attrac- 
tion of the table-land alone. 
The general conclusion then is this. In all cases, the real disturbance will be less 
than that found by computing the effect of the mountains, on the law of gravitation. 
Near to the elevated country, the part which is to be subtracted from the computed 
effect is a small proportion of the whole. At a distance from the elevated country, 
the part which is to be subtracted is so nearly equal to the whole, that the remainder 
may be neglected as insignificant, even in cases where the attraction of the elevated 
country itself would be considerable. But in our ignorance of the depth at which the 
downward immersion of the projecting crust into the lava takes place, we cannot 
give greater precision to the statement. 
In all the latter inferences, it is supposed that the crust is floating in a state of 
equilibrium. But in our entire ignorance of the modus operandi of the forces which 
have raised submarine strata to the tops of high mountains, we cannot insist on this 
as absolutely true. We know (from the reasoning above) that it will be so to the 
limits of h'eahage of the table-lands; but within those limits there maybe some range 
of the conditions either way. It is quite as possible that the immersion of the lower 
projection in the lava may be too great, as that the elevation may be too great ; and 
in the former of these cases, the attraction on the distant stations would be negative. 
Again reverting to the condition of breakage of the table-lands, as dominating 
through the whole of this reasoning, it will be seen that it does not apply in regard 
to such computations as that of the attraction of Schehallien and the like. It 
applies only to the computation of the attractions of high tracts of very great hori- 
zontal extent, such as those to the north of India. 
Royal Observatory, Greenwich, 
January 19, 1855. 
