of Latitudes for- Loco. I Attraction , 321 



supposing them completed, should regard their employment for 

 the purpose of correcting the latitudes as injurious for the ob- 

 ject of arc-measurement ; then it certainly becomes our duty 

 to justify such a difference of opinion by substantial reasons. 

 We will proceed therefore to perform this duty, and hope that 

 the arguments which are most important to the maintenance 

 of our opinion will not fail in their effect upon the mind of the 

 unprejudiced reader. 



" Our opinion is this : — In all the preceding arguments one 

 important circumstance is entirely left out of consideration, 

 one assumption is constantly and tacitly made, which is in- 

 deed very necessary as affording these arguments some justi- 

 fication, but which yet in itself lacks all justification. The 

 assumption is this — that between that portion of the whole 

 disturbance which, as we said before, we are able to take into 

 account, and that other portion which, for want of information 

 we must disregard, there is no necessary connexion. If this 

 assumption is well founded, if the whole disturbance is nothing 

 but a lawless aggregate of fortuitous individual forces coope- 

 rating, then we cannot deny that the consideration of any one 

 of them accessible to us would be a real gain ; but if this as- 

 sumption is not admissible, all inferences from it fall to the 

 ground. That this really is the case (that is to say, that in 

 judging of the admissibility or non-admissibility of the correc- 

 tions, the assumption we have pointed out is the essential one) 

 will be evident if we turn again to the examples just given. The 

 true and only foundation of the absolute consent given to the use 

 of the correction in the case of the pyramid consists in the fact 

 that we know, in this case with certainty, that the disturbing 

 mass considered is one that has been brought from a distance 

 and placed there ; so that there can be no question of any con- 

 nexion between it and the surrounding masses on or under the 

 surface of the earth. It is different, however, in both the 

 other cases. Apart from the difficulties which are asso- 

 ciated with the exact measurement of the geometrical form 

 and solid content of the disturbing surface inequalities, the 

 fullest confidence in the result of geological research must 

 here be especially needed in order to repel the doubt whether 

 certain counteracting effects must not necessarily be con- 

 nected with the undeniable operation of these visible masses. 

 This doubt seems all the more justifiable when one considers 

 how comparatively insignificant are even the greatest depths 

 at which any practical knowledge of the conformation of the 

 earth's crust has been obtained; and when, further, it is remem- 

 bered that even these few actual investigations can very sel- 

 dom be undertaken at the actual spot, but that we must in 



