ATTRACTION IN THE CASE OF THE ENGLISH ARC. 
35 
order to represent to the eye the effects produced by local attraction. The parallel 
dotted lines are the positions which the verticals at the several stations would assume, 
when corrected for local attraction, if the arc were bent up into a straight line at right 
angles to the vertical at Arbury Hill, which is the station nearest to the middle of the 
arc. The other lines passing through the stations of the arc show the direction and 
relative magnitude (the actual magnitude would be hardly discernible) of the de- 
flection of the plumb-line at the other five stations relatively to that at Arbury Hill. 
This diagram shows that the plumb-line at Arbury Hill and Greenwich is nearly 
equally affected, and that on the Arbury side of Blenheim there must be more 
attracting matter than on the Greenwich side. At Clifton the plumb-line is drawn 
much to the south, and still more so at Burleigh Moor, as might be expected from 
its proximity to the sea. 
8. The chief peculiarity is in the arc between Dunnose and Greenwich. As Dun- 
nose is on the south coast of the Isle of Wight, it might have been (a jonori) expected 
that the plumb-line would have been drawn considerably northward, so as to increase 
the amplitude between that place and Greenwich, whereas it is diminished. This 
peculiarity must arise from the character of the immediate neighbourhood of Dun- 
nose and the form of the coast there. That there is some peculiar arrangement of 
the mass in the neighbourhood of Dunnose, more lying on the south of its parallel 
than on the north, appears from the table of differences of amplitude between South- 
ampton and Dunnose, Boniface Down, Week Down, and Port Valley, given at p. xl 
of the Introduction to Captain Yolland’s ‘Astronomical Observations made with 
Airy’s Zenith Sector,’ published in 1852. The following is the Table alluded to: — 
Amplitudes. 
G— A. 
Geodetical. 
Astronomical. 
Southampton and Dunnose 
Southampton and Boniface Down ... 
Southampton and Week Down 
Southampton and Port Valley 
Southampton and Black Down 
0° 17 43-24 
0 18 37-36 
0 18 56-71 
0 19 1*58 
0 13 36-69 
0 17 39-53 
0 18 36-13 
0 18 55-26 . 
0 19 1-16 
0 13 37-79 
-P3-71 
-f 1-23 
-1-1-45 
-1-0-42 
— 1-10 
F 2 
