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III. On the Computation of the Effect of the Attraction of Mountain-masses, as dis- 
turbing the Apparent Astronomical Latitude of Stations in Geodetic Surveys. 
By G. B. Airy, E,sq., Astronomer Royal. 
Received January 25, — Read February 15, 1855. 
A PAPER of great ability has lately been communicated to the Royal Society by 
Archdeacon Pratt, in which the disturbing effects of the mass of high land north- 
east of the valley of the Ganges, upon the apparent astronomical latitudes of the 
principal stations of the Indian Arc of Meridian, are investigated. It is not my in- 
tention here to comment upon the mathematical methods used by the author of that 
paper, or upon the physical measures on which the numerical calculation of his for- 
mulae is based, but only to call attention to the principal result ; namely, that the 
attraction of the mountain-ground, thus computed on the theory of gravitation, is 
considerably greater than is necessary to explain the anomalies observed. This sin- 
gular conclusion, I confess, at first surprised me very much. 
Yet, upon considering the theory of the earth’s figure as affected by disturbing 
causes, with the aid of the best physical hypothesis (imperfect as it must be) which 
I am able to apply to it, it appears to me, not only that there is nothing surprising 
in Archdeacon Pratt’s conclusion, but that it ought to have been anticipated ; and 
that, instead of expecting a positive effect of attraction of a large mountain mass upon 
a station at a considerable distance from it, we ought to be prepared to expect no 
effect whatever, or in some cases even a small negative effect. The reasoning upon 
which this opinion is founded, inasmuch as it must have some application to almost 
every investigation of geodesy, may perhaps merit the attention of the Royal Society. 
Although the surface of the earth consists everywhere of a hard crust, with only 
enough of water lying upon it to give us everywhere a couche de niveau, and to 
enable us to estimate the heights of the mountains in some places, and the depths of 
the basins in others ; yet the smallness of those elevations and depths, the correctness 
with which the hard part of the earth has assumed the spheroidal form, and the 
absence of any particular preponderance either of land or of water at the equator as 
compared \^ith the poles, have induced most physicists to suppose, either that the 
interior of the earth is now fluid, or that it was fluid when the mountains took their 
present forms. This fluidity maybe very imperfect; it may be mere viscidity ; it 
may even be little more than that degree of yielding which (as is well known to 
miners) shows itself by changes in the floors of subterraneous chambers at a great 
depth when their width exceeds 20 or 30 feet; and this yielding may be sufficient for 
MDCCCLV. P 
