357 
of Edinburgh, Session 1867 - 68 . 
dian for a standard of measure,— it has certainly not become a 
common practice among nations, nay I doubt whether a single 
people has adopted it : and extraordinary in the second place, be- 
cause, if such degrees of the meridian can be measured with 
“ almost perfect mathematical exactitude, ” — the length of the 
earth’s axis can be computed from them to the same proportional 
amount of most remarkable perfection ; instead of remaining as 
the Proceedings' author says it does remain, “ an unknown and un- 
“ determined linear quantity.” 
The question arises, therefore, why has the calculation not been 
made? And the answer must be— -that no one but the Proceedings 
author knows of any such marvellous data for the purpose: and he is 
either so inexpert at the computation that he cannot, or so careless 
of the state of geodesical knowledge in his time, that he does not 
choose to, make the very great improvement, which his statement 
implies that he might do, in our physical knowledge, or in our idea 
of the exact length of the earth’s polar axis, — the most important 
base which men can refer to for measuring the sizes and distances 
of the sun, planets, and stars. 
No one else, I repeat, knows of these almost absolutely perfect 
data. Such earth-axial knowledge as that represented by the last- 
volume of the Ordnance Survey, is based certainly upon the 
measured lengths of arcs of the meridian, -—but not depending 
on measurements of one degree only. On the contrary, there is a 
class of errors concerned, whose proportional effects increase so 
rapidly, with the smallness of the arc, that the tendency of the age 
has long been to measure an arc of as many degrees as possible, 
instead of only one degree. Hence, both in India, and Russia, 
Britain, France, and Germany, the arcs measured have been made 
to contain even 10°, 30°, and 60°, by extending them, where pos- 
sible, across neighbouring countries. Thus the Indian arc, begun 
near Cape Comorin, is being pushed northward over the Himalayas. 
The Russian meridian arc, commenced at North Cape, has crossed 
the Danube, and is approaching the Mediterranean ; while all the 
states of Europe are at this moment engaged in a grand arc of 
parallel, stretching from Yalentia in Ireland to the Ural Moun- 
tains in Russia. 
Yet though both Captain Clarke’s and other geodesists’ results 
