316 MM. Struve and Dollen on the Correction 



lation which covers Great Britain and Ireland has been treated 

 as one connected -whole ; and, after rigorous reduction, has 

 served in its entirety for determining the distances between 

 the parallels corresponding to the astronomical points. Now, 

 although (as the author himself remarks) a close adherence of 

 the triangulation to the meridian is not demanded for the 

 determination of the parallels, in the present case, also, the 

 great number of carefully determined azimuths and their 

 agreement with one another, exclude any suspicion that inac- 

 curacy is introduced into the work by the treatment actually 

 adopted. On the other hand, the actual arrangement offers 

 the very important advantage that a much greater number of 

 accurately determined latitudes are thus brought into com- 

 putation ; whereby it happens that, while other arcs as a rule 

 have only one latitude to each two degrees, here there are 

 two or three on an average to each single degree. It is this 

 great number of latitudes brought into the calculation which 

 gives to the arc of the English meridian more importance, 

 for the determination of the figure of the earth, than would be 

 due to its mere extent in comparison with other measures ; 

 as thereby the possible influence both of local deflections of 

 the plumb-line and of inevitable small errors in observation, 

 upon the result to be obtained, is considerably reduced. 



We find also, for our part, in the English work, a substan- 

 tial gain in the fact that, in the determination of the figure of 

 the earth by combining together all existing trustworthy arc- 

 measures, those equations of condition are also brought in 

 which express how much the deduced dimensions are affected 

 by recognizing as subject to error the assumed relation of the 

 standards employed in the different measurements. The 

 actual magnitude of the errors of this kind can be discovered 

 only by direct comparison of the different linear standards. 

 This work of comparison has been performed with great care 

 at Pulkova, for almost all known arcs. The table of the re- 

 sults of such direct comparisons, the completion of which, as 

 given in the account of the Russo-Scandinavian arc, cost much 

 time and trouble, and the importance of which has been recog- 

 nized by their admission as aforesaid by the English geodesists, 

 should certainly therefore be regarded as a distinct merit in the 

 Russian work, 



But Gen. Schubert considers the special merit of the English 

 work to reside in something quite different from what we have 

 mentioned. He regards it as a step in advance which marks 

 an era, that the experiment has here been for the first time 

 made of correcting each latitude, before it is used for deducing 

 the figure of the earth, for the effect of the undoubtedly exist- 



