FROM THF ORDNANCE TRIGONOMETRICAL SURVEV. 
609 
The difference between the measured and computed length of the Misterton Carr 
base, near Doncaster, also measured with the steel chains, is only O' 191 ft,, or about 
2 inches; and it will be observed that the difference between the computed and 
measured lengths of these three bases (tneasured with chains) is not greater than the 
difference between the measured and computed length of the Lough Foyle and Salis- 
bury Plain bases (measured with the compensation bars), from which it may be in- 
ferred that bases measured with steel chains are deserving of the greatest confidence; 
and when the great simplicity, portability, and cheapness of the chains is compared 
with the complex, heavy and expensive apparatus of the compensation bai’S, I should 
anticipate that they would be more generally employed than they have been of late 
years, especially in the colonies, and in countries where the transport of heavy arti- 
cles is effected with difficulty. 
The length of the base on Rhuddlan Marsh in North Wales, which was measured 
with steel chains, differs l'596ft. from the computed length; but from the circum- 
stance that the extremities of the base are very badly situated with reference to the 
surrounding Trigonometrical stations, the angles being very acute, and not well ob- 
served, we have placed little confidence in the result of the comparison of its com- 
puted and measured length. 
One of the first practical results arising from the completion of the triangulation 
is, that we are now able to engrave the latitude and longitude on the marginal lines 
of the old sheets of the 1-inch map of England, and this is now being done. 
The following account of the Trigonometrical operations and calculations has been 
drawn up by Captain Alexander R. Clarke, R.E. : this account may be considered 
as an abridgement of that more detailed account which is now in the press, and will 
be shortly published. 
It will be seen that the equatorial diameter of the earth, as derived from the 
Ordnance Survey, is 7926'610 miles, or about one mile greater than it is given by the 
Astronomer Royal in his ‘ Figure of the Earth,’ and that the ellipticity is ^^ 3 , or as 
the Astronomer Royal conjectured, something “ greater than - 3 ^ 05 ” which he gives in 
the same paper. 
The mean specific gravity of the earth, as derived from the observations at Arthur’s 
Seat, was stated in a former paper to be 5*14 ; the calculations have since been revised, 
and we now find it to be 5'316. 
The mean specific gravity of the earth, as derived from the only other observations 
on the attraction of mountain masses on which any reliance has been placed, viz. the 
Schehallien observations, give, as finally corrected by Hutton, or almost 5*0. 
From the experiments with balls we have the following results : — 
By Cavendish, as corrected by Baily .... 5'448 
By Baily 5'67 
By Reich 5'44 
4 L 2 
