of an Arc of the Meridian. 49 1 
more than 10"; and that too from the effects of attraction in a 
southern direction, where the deficiency of matter would lead 
us to believe the reverse would happen . 
I am perfectly aware that it is possible to state a case, in 
which the plumb-line of a sector would deviate from the true 
vertical by such a quantity. Thus, for instance, in a chalky 
county, like the southern part of the kingdom, if the instrument 
were set up adjoining the terminations of two strata running 
east and west, one of chalk and the other of much denser ma- 
terials, the effect would be as we have found it. But, at Dunnose, 
this argument does not apply; nor is there reason to believe, from 
external appearances, that it will do so, with regard either to 
Arbury Hill or the northern extremity of the meridional line. 
It was the discovery of the disagreement between the subtense 
in the heavens, of the whole arc, and its corresponding terrestrial 
one, with those of its parts, which led me to apply to his Grace 
the Duke of Marlborough, for the observations made at 
Blenheim on y Draconis, or some other star. His Lordship’s 
compliance with my request, is shown, from the Table of results, 
to be serviceable ; as the arc contained between the observatory 
at Blenheim and Dunnose, deduced from his Grace’s observa- 
tions, and those made at the latter place, with the meridional 
distance 446498 feet, give 60890 fathoms, for the length of the 
degree on the meridian in latitude 51 0 13'; which agrees nearly 
with the length of the degree at the middle point between 
Greenwich and Dunnose. However, under all considerations of 
the means by which the degree in 51 0 13' has been obtained, I 
am inclined to believe there is an uncertainty in it, of 6 or 7 
fathoms, answering to about §•" in latitude. 
But, if the measured space between his Grace’s observatory and 
