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X. Abstract of the Results of the Comparisons of the Standards of Length of England , 
France , Belgium , Prussia, Russia, India, Australia, made at the Ordnance Survey 
Office, Southampton. By Captain A. R. Clarke, R.F., F.R.S., Ac., under the 
Direction of Colonel Sir Henry James, R.F., F.R.S., Ac., Director of the Ordnance 
Survey. With a Preface by Colonel Sir Henry James, R.F., F.R.S., Ac. 
Received November 15, — Read December 13, 1866. 
The principal triangulation of the United Kingdom was finished in 1851 ; and the tri- 
angulations of France, Belgium, Prussia, and Russia were so far advanced in 1860 that, 
if connected, we should have a continuous triangulation from the Island of Valentia, on 
the south-west extremity 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 extremities of this arc, and thus obtain a crucial test 
of the accuracy of the figure and dimensions of the earth, as derived from the measure- 
ment of arcs of meridian, or the data for modifying the results previously arrived 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 triangu- 
lation 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 Government 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 work 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 
the exact same stations. 
The results obtained by the French geometricians is published in the Supplement to 
vol. ix. of the ‘Memorial du Depot General de la Guerre,’ 1865, and the agreement 
with the results obtained by the English is truly surprising. 
But however accurately the trigonometrical observations might be performed, it is 
obvious that, without a precise knowledge of the relative lengths of the standards used 
* Extension of the Triangulation of the Ordnance Survey into France and Belgium. London, 1863. 
MDCCCLXVII. • Z 
