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. 



6. 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. 



