150 
Local Attraction. 
[No. 2, 
fear will never be met; that is, To devise a method for determining 
the absolute latitude of some one place included in the map. The 
state of the question is, as I have said, at present this: the position 
of places determined by geodetic operations is correct and free from 
the effect of local attraction, relatively to the station from which the 
operations start. But how to find the latitude of this starting point, 
freed from the errors produced by local attraction, is a problem un¬ 
solved, and unlikely to be solved. Even if any spot exists which is 
altogether free from local attraction, that is where all such influences 
nullify each other, it is impossible to discover it and to assure our¬ 
selves of the fact. 
G. Thus geodesy can give us accurate maps of the relative position 
of places ; but cannot, with the same accuracy, assign the position 
of the maps on the terrestrial spheroid. Suppose, to take a compre¬ 
hensive case, that the whole globe were surveyed and all places in it 
connected by triangulation with the spot in the north where the plumb- 
line points to the north-pole in the heavens. The positions of all places 
would be found free from error relatively to this spot—which is 
commonly called the North Pole of the earth. But how can we be 
sure that the plumb-line at that spot is hanging in the true vertical ? 
It may be under the influence of local attraction: in which case, 
although it points to the pole in the heavens, the spot in question 
will not be the pole on the earth. There is no means, nor can I 
conceive any means possible, short of ascertaining all the disturbing 
causes throughout the earth’s mass and calculating their effects, of 
determining whether the plumb-line is or is not at the true pole. 
The accurate position, therefore, of our maps on the terrestrial 
spheroid which depends upon this question is alike unknown and 
uncertain. This is the point to which the investigation is brought, 
and where, I have no doubt, it will stop. It is satisfactory that the 
mapping of a country may be laid down, free from all error as to the 
relative situation of places: also that the relative amount of local 
attraction, comparing one place with another, can be determined, 
because this may assist in ascertaining the structure of the crust 
below. It would, however, be still more satisfactory if this one re¬ 
maining difficulty could be removed, as it would make the data more 
complete for the high scientific determination of the Figure of the 
Earth. 
