1866.] On the Results of Comparisons of Standards of Length,' 311 



II. Abstract of tlie Eesults of the Comparisons of the Standards 

 of Length of England, I'rance, Belgium, Prussia, Russia, 

 India, Australia, made at the Ordnance Survey Office, South- 

 ampton/^ By Captain A. E. Clarke, E.E,, F.B.S., &c., under 

 the Direction of Colonel Sir Heney James, E.E., F.R.S., &c.. 

 Director of the Ordnance Survey, Received November 15, 1866. 



(Abstract.) 



In the preface to this paper, Sir Henry James gives an account of the 

 circumstances under which the work was undertaken, as follows. (A 

 Table of results is appended, p. 313.) 



The principal triangulation of the United Kingdom was finished in 

 1851 ; and the triangulation s of France, Belgium, Prussia, and Russia 

 were so far advanced in 1860, that, if connected, we should have a conti- 

 nuous triangulation from the Island of Yalentia on the south-west ex- 

 tremity of Ireland, in north latitude 51° 55' 20", and longitude 10° 20' 40" 

 west of Greenwich, to Orsk on the River Ural in Russia. 



It was therefore possible to measure the length of an arc of parallel 

 in latitude 52° of about 75°, and to determine, by the assistance of the 

 electric telegraph, the exact difference of longitude between the extre- 

 mities of this arc, and thus obtain a crucial test of the accuracy of the 

 figure and dimensions of the earth, as derived from the measurement of 

 arcs of meridian, or the data for modifying the results previously ar- 

 rived at. 



The Russian Government, therefore, at the instance of M. Otto Struve, 

 Imperial Astronomer of Russia, invited (in 1860) the cooperation of the 

 Governments of Prussia, Belgium, France, and England, to effect this 

 most important object, and to their great honour they all consented, and 

 granted the necessary funds for the execution of the work. 



The portion of the work which was assigned to me, was the connexion 

 of the triangulation of England with that of France and Belgium, and I 

 published the results of this operation in 1862 But this work has been 

 done in duplicate ; for when application was made to the French Go- 

 vernment to permit the necessary observations to be made in France, they 

 not only consented to allow this, but at the same time volunteered to join 

 in the labour and expense of the v/ork itself. 



It would obviously have been wrong to mix up observations made with 

 different kinds of instruments and on different principles, and therefore it 

 was agreed that the work should, in fact, be made in duplicate, both the 

 French and English geometricians using exactly the same stations. 



The results obtained by the French geometricians is published in the 



* Extension of the Triangulation of the Ordnance Survey into France and Bel- 

 gium. London, 1863. 



