1865.] On Local Attraction, 41 
Postscript, 
[Received 29th April, 1865. | 
If the raw or uncorrected results of the Surveys in India and 
Europe (I mean uncorrected for local attraction) are made use of, 
they bring out meridians of a slightly different curvature in these 
different parts of the earth. If these were the true forms of the seve- 
ral meridians the result would be that the equator could not be a circle 
and the figure of the earth not a spheroid of revolution. A few years. 
ago, General T. F. de Schubert calculated the form of an ellipsoid of 
three unequal axes which would best suit the observations. Captain 
Alexander Clarke, R. E. (Memoirs Roy. As. Soc’ Vol. XXIX, for 
1860,) went through the same calculation, following Bessel’s method. 
His result was that the equatorial radius in longitude 14° or there- 
abouts is one mile longer than that in longitude 104°. He speaks 
with hesitation regarding the result, on the ground that the data are 
far too scanty to lead to a conclusion to be relied upon. He appears, 
however, not to shrink from the hypothesis on which he works, from 
the true grounds of distrust, viz. (1) the @ prior? improbability 
that the earth’s mean figure is not one of revolution, as the evidence of 
the fluid-origin of that figure is overwhelming* and (2) that the effect 
of local attraction is altogether overlooked by him. General de Schubert 
indeed in a subsequent paper (See Monthly Notices of Royal Astrono- 
mical Soc. for 1860, p. 264, where it is noticed) does anticipate that 
local attraction may modify and altogether destroy the data on which 
he rested the argument of an ellipsoidal figure. The Paper which I 
have sent to the Society and have noticed in this letter gives, for the 
first time, a method for estimating the effect of local attraction and 
proves (in the third section) that so very moderate an allowance as 1” 
or 2” for local attraction will altogether destroy the disparity between 
the curvature of the different meridians. When the arguments in this 
paper are impartially weighed I feel convinced that the improbable 
ellipsoidal theory will be abandoned altogether. 
* The evidence, with full details, is given in the third edition of my treatise 
on the “ Figure of the Harth” now passing through the press at Cambridge 
and a copy of which when published I purpose sending to the Society. 
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