1865.] On Local Attraction. 35: 
M. Otto Struve has lately called attention to similarly important 
deflections caused by local attraction in Russia—and especially to a 
remarkable difference of deflection at two stations near Moscow, only 
about eighteen miles apart, which is attributed to an invisible 
unknown cause in the strata below. 
It has become, therefore, an important inquiry :—What degree of 
uncertainty does Local Attraction, if not allowed for, introduce into 
the two problems of geodesy, viz. (1) obtaining correct Maps of any 
country, and (2) determining the Mean Figure of the Earth. 
These matters are discussed in the present Paper; and I would 
here observe, that the paper is complete in itself, and does not require 
a study of the previous communications. 
2. With regard to the construction of Maps from Survey operations 
I show, as before in India, that no map in any other part of the 
world will be affected except in the way already stated, if the length 
of every measured arc of latitude is not greater than twelve degrees. 
and a half, and of every measured are of longitude not greater than 
fifteen. Now in point of fact, however long the great ares (such as 
the Anglo-Gallic, the Russian, and the Indian) may be, they are 
always broken up into much smaller portions, so as to bring them 
very far within the above-mentioned limits. Hence the maps 
constructed from geodetic operations will always be relatively correct 
in themselves ; but the precise position of the map on the terrestrial 
spheroid will be unknown by the amount of the unknown deflection 
of the plumb-line in latitude and longitude at the place which fixes 
the map. 
In India the effect of the Himalaya Mountains and the Ocean, 
taken alone, would throw out the map by nearly half a mile. And, 
as already stated, there is no way of discovering with certainty how 
much this is increased or diminished by the effect of variations of 
density in the crust. If, however, the calculations which I give in 
the third section of this Paper are accepted, they show that the effect 
of variations in the density of the crust below almost entirely 
counteracts that of the mountains and ocean at Damargida in 
latitude 18° 3’ 15”, and the displacement of the map is almost 
insensible if fixed by that station. If fixed by the observed latitude 
of any other station, the map will be out of its place by the local 

