1 PUOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



System of the Cote d'Or. 



System of the Western Alps. 



System of Corsica and Sardinia. 



Systeni of the North of England. 

 I have already explained the reason why a considerable number of 

 the circles of reference of the European systems, as determined by 

 our author, must necessarily intersect each other near a common 

 point, and I have also explained (p. xxxviii) the uncertainty of the posi- 

 tion of the centre of reduction of each of these circles. That centre, 

 it will be recollected, was assumed for each particular system. The 

 ten circles of reference for the above ten systems, as previously deter- 

 mined, pass near to the point D, and they are now assumed to pass 

 accurately through that point, which thus becomes a common centre 

 of reduction for them all. In accordance with this assumption, each 

 circle has been moved laterally from its position, as previously deter- 

 mined, through an angular space in general small, but in the system 

 of the Rhine amounting to about 3° or 4°, and in that of the north of 

 England to the amount, I conceive, of 7° or 8°. In these displace- 

 ments there is nothing inconsistent with the previous approximate 

 determinations of the positions of these great circles. The author 

 regards them only as provisional, but after having assumed the ten 

 great circles of reference above-mentioned to pass accurately through 

 the pentagonal centre D, their positions, it must be remembered, are 

 no longer provisional, except so far as the position of the whole 

 reseau on the earth's surface is so ; and, moreover, unless the posi- 

 tion now assigned to the reseau be very approximately correct, the 

 whole evidence at present adduced in favour of the theory is value- 

 less ; for that evidence rests entirely on that approximate coincidence 

 of lines of the reseau with actual lines of elevation, which would be 

 totally destroyed by any but very small displacements of the whole 

 reseau. 



Now, the impossibility of altering materially the positions of the 

 great circles of reference of the European systems, determined as 

 they have been from the facts afforded by a limited region, appears 

 to me to lead to very important inferences, to which our author has 

 not, I think, made any distinct allusion. We may allow the direc- 

 tions of these great circles within the limited region of Europe to be 

 determined, liable to an error not exceeding 3° or 4° ; but I have 

 already explained that their assigned positions within that region (as 

 depending on their centres of reduction) may be liable to a much 

 greater error (p. xxxvii). Now, referring more especially to the ten 

 great circles of reference above mentioned as passing through the 

 pentagonal centre D, the evidence deducible from them in favour of 

 our author's theory depends entirely on their passing, if not exactly, at 

 least very approximately through that point, the amount of admissible 

 deviation from such position being much less than the error to which, 

 as above explained, the positions at present assigned to them are 

 liable. For if these deviations were not very small, the great circles 

 of reference could not be referred to great circles of the reseau pass- 

 ing through D as their proper representatives, and the evidence 



