36 
ARCHDEACON PRATT ON THE EFFECT OF LOCAL 
For the position of these places see Plate II. Three of them are in the Isle of 
Wight, south of the parallel of Dunnose. The Table shows that the plumb-line at 
Dunnose is affected in the southerly direction with reference to each of these three 
places, Boniface Down, Week Down, and Port Valley, to a considerable degree, 
viz. 2"'48, 2"'26, and 3"‘29 ; and therefore there must be some large mass in the im- 
mediate neighbourhood of Dunnose Station between its parallel and that of Boniface 
Down. The Table show's that the amplitude between the parallel of Southampton 
and Black Down on the Dorsetshire coast is increased — as we should expect. This 
confirms the supposition that there is some peculiarity in the Isle of Wight south of 
Dunnose, which an actual geographical survey can alone determine. 
9. The calculation I have gone through, in the last article but two, shows that the 
amplitude of the whole arc between Dunnose and Burleigh Moor is increased by 
local attraction to the amount of 4"'722. 
10. This discussion of the arc between Dunnose and Burleigh Moor suggests the 
importance of obtaining, in the best way we can, the amount of local attraction at 
the several stations of the arc by some direct means, that the corrections may be 
applied to the amplitudes before they are used in the Problem of the Figure of the 
Earth. For although these errors in the amplitudes are rendered less injurious to 
the result by comparing the arc with other arcs separated from it considerably 
in latitude, an arc which per se leads to so unusual an ellipticity cannot be so safely 
employed in the general problem, as when it is freed from the source of error which 
seems to lead to that ellipticity. 
11. It may in the end appear, even after the corrections for local attraction are 
applied, that the curvature of the English arc is different from the mean curvature, 
and, as I have stated in my foianer |)aper, the science of geology w'ould tend rather 
to favour such a conception. But even in that case, the values of the ellipticities 
of the separate portions of the arc must present a much more uniform appearance 
than those deduced in this paper from the present data (see art. 4.). 
I proceed now to the second part of this communication, to obtain a formula for 
calculating the attraction. 
II. Investigation of a Formula for calculating the amount of Local Attraction on any 
Station. 
12. In the former paper I obtained a formula for calculating the attraction by 
supposing the superficial mass cut into lanes by great circles passing through the 
attracted station, and the limes divided into compartments according to a certain 
law. 'File formula thence deduced may be applied to attraction in England. But I 
will now deduce another foi niida which in some cases may be found more con- 
venient in practice. Tlie former tnethod takes account of the curvature of the earth, 
and determines the attraction of any masses lying anywhere above the sea-level 
lietween the given station and the antipodes. In the present instance I shall suppose 
