in the length of the pendulum vibrating seconds. 425 
tible, and at the short distance of London, this additional force 
is no longer sensible. From its intensity, and the limited 
sphere of its action, it might perhaps be inferred that the dis- 
turbing material is of considerable density, and not very 
distant from the surface. 
It must be evident that nothing very decisive respecting the 
general elli pticity of the Meridian can be deduced from the 
present experiments. For this purpose it is requisite that the 
extreme stations should comprise an arc of sufficient length 
to render the effect of irregular attraction insensible ; and this 
effect might be diminished, if not wholly prevented, by select- 
ing stations of similar geological character, and which should 
differ as little as possible in elevation above the level of the sea. 
If however some deduction be made for the superior den- 
sity which it has been remarked exists at Portsoy, the com- 
pression j~ deduced from that station and Unst, may perhaps 
be considered as not far distant from the truth, both being 
situated on rocks of a similar nature ; Unst consisting chiefly 
of serpentine, and Portsoy, of serpentine, slate, and granite ; 
and as the ellipticity given by the experiments at Unst 
and Arbury Hill, is nearly the same as that resulting from 
Unst and Portsoy, it would be no improbable conjecture that 
the sudden increase of gravitation observed at Arbury Hill, 
may be occasioned by a rock of primitive formation, approach- 
ing the surface of the earth in the vicinity of that station.* 
These facts appear sufficient to explain the anomalies which 
* Since the above was written, I find the conjecture I have hazarded remarkably 
supported by fact; for on consulting Smith’s Geological Map of England, it appears 
that Mount Sorrel, a mass of granite, is situated, together with other rocks of primitive 
formation, about 30 miles to the north of Arbury Hill. 
3 l * 
MDCCCXIX 
