568 
Mr. Airy on the 
Dunkirk : 4. The English arc from Dunnose to Clifton : 
5. The Swedish arc from Mallorn to Pahtavara. 
The Peruvian arc I have taken as recalculated by Delambre 
in the Base du Systeme Metrique, Tome iii. p. 112, & c. I 
have supposed the toise of Peru equal to 1,065766 fathoms. 
The Indian arc is taken as finally corrected by Colonel 
Lambton in the Philosophical Transactions for 1823. 
In the estimation of the length of a part of the French arc, 
which Colonel Lambton has given in the Philosophical Trans- 
actions for 1818, and which Captain Kater has sanctioned by 
copying without any remark, there appears to be a serious 
error, arising from an unnecessary reduction for temperature. 
As I am sensible that I am now opposed to two Gentlemen 
whose assertion on such a point is almost decisive, I will 
state distinctly the reasons which have led me to this con- 
clusion. 
In the measure of the base at Melun (Base du Systeme 
Metrique, Tome ii. p. 44), the length of the base was re- 
duced to the length which it would have had if the measuring 
rods had been used at the temperature of 13 0 of Reaumur, 
or 1 6°£ centigrade. The number of rods employed was 
four ; but the rod or module No. 1 was compared with each 
of the others, and the base was expressed by the length of 
the module No. 1 at the temperature of 16°^ centigrade. 
From the logarithm set down in p. 698, it appears that this 
was the length used in the calculation of the triangles. 
The module No. 1 was found at this temperature to be less 
than two toises (the toise being the length of an iron standard, 
called the toise of Peru, at the temperature of 1 3 0 of Reaumur) ; 
but from an error in the zero point of the metallic thermo- 
