312 



Capt. A. R. Clarke on the Results of the [Dec. 13, 



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 per- 

 formed, it is obvious that, without a knowledge of the exact relative 

 lengths of the standards used as the units of measure in the tiiangulatioii 

 of the several countries, it would be impossible accurately to express the 

 length of the arc of parallel in terms of any one of the standards. 



It was therefore necessary that a comparison of the standards of length 

 should be made, and as we had a building and apparatus expressly erected 

 for the purpose of comparing standards at this Office, the English Govern- 

 ment, on my recommendation, invited the Governments of the several 

 countries named to send their standards here, and we have had the fol- 

 lowing compared with the greatest accuracy : — ■ 



1 . Russian standard, double toise, P. 



2. Prussian standard toise. 



3. Belgium standard toise. 



4. Platinum metre of the Royal Society, compared with the standard 

 metre of France by M. Arago. 



5. Enghsh standard yards, A, B, C, 29, 47, 51, 55, 58. 



6. Ordnance Survey 10-foot standard bar. 



7. Indian 10-foot standard bars, new and old. 



8. Australian 10-foot standard bar. 



9. In addition to the above, the 10-foot standard bar of the Cape of 

 Good Hope was compared heie in 1844. 



We have invited the Governments of Austria, Spain, and the United 

 States of America, also to send their standards. "We have been promised 

 that of Austria, and, but for the unfortunate war in which she has been 

 lately engaged, we should have received it before this. 



I have entrusted the execution of the work of comparison and the 

 drawing up of the results to Captain Alexander R. Clarke of the Royal 

 Engineers, who designed the apparatus used. The numerous compari- 

 sons to be made entailed a great amount of labour upon him and his 

 assistants, Quartermaster Steel and Corporal Compton, of the Royal 

 Engineers. 



Before the connexion of the triangulation of the several countries into 

 one great network of triangles, extending across the entire breadth of 

 Europe, and before the discovery of the electric telegraph and its exten- 

 sion from Valentia to the Ural Mountains, it was not possible to execute 

 so vast an undertaking as that which is now in progress. It is, in fact, 

 a work which could not possibly have been executed at any earlier period 

 in the history of the world. The exact determination of the figure and 

 dimensions of the earth has been the great aim of astronomers for up- 

 wards of two thousand years, and it is fortunate that we live in a time 

 when men are so enlightened as to combine their labours to effect an ob- 



