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IV. Oh the Effect of Local Attraction upon the Plumb-line at Stations on the English 
Arc of the Meridian, between Dunnose and Burleigh Moor ; and a Method of 
computing its Amount. By the Venerable John Henry Pratt, M.A., Archdeacon 
of Calcutta. Communicated by the Rev. J. Challis, M.A., F.R.S. 8fc. 
Received June 5, — Read June 21, 1855. 
1. In a former communication I endeavoured to point out a method for calculating 
the deflection of the plumb-line at stations on the Indian arc, caused by the attraction 
of the Himalayas and the vast regions beyond, with a view to the correction of the 
astronomical amplitudes of the measured subdivisions of the arc, before they are 
applied to the determination of the ellipticity of the earth. 
The same subject is taken up in the present paper, but in reference to one of the 
English arcs, that between Dunnose and Burleigh Moor; and a different method of 
calculating the attraction is given. 
1. Calculation of the Ellipticity of the English Arc between Dunnose and Burleigh 
Moor, without taking account of Local Attraction. 
2. The data for this calculation are taken from Mudge’s ‘Trigonometrical Survey 
of England,’ vols. ii. iii., and are as follows : — 
Arc between 
Amplitude. 
Arc in feet. 
Latitude of middle point. 
1. Dunnose and Greenwich 
2. Greenwich and Blenheim 
3. Blenheim and Arbury Hill 
4. Arbury Hill and Clifton 
5. Clifton and Burleigh Moor 
o / // 
0 51 31-39 
0 21 47-90 
0 23 0-30 
1 14 3-40 
1 6 50-11 
313696-0 
132802-0 
139822-0 
450045-3 
406462-9 
51 2 53-316 
51 39 33-316 
52 1 57-831 
52 50 29-580 
54 0 56-335 
Captain Kater has shown, by an examination of the scale employed, that the lengths 
of the arcs in feet should all be corrected by multiplying by O’OOOO/ and adding the 
results ; but as I shall use only the ratios of these arcs to each other, this correction 
need not be applied. 
3. From these data I now proceed to compare these five subdivisions of the arc 
between Dunnose and Burleigh Moor, two and two, and thence deduce the ten values 
of the ellipticity which the ten combinations will give, and the arithmetic mean of 
them, which will be a fair representation of the mean ellipticity of the whole arc, upon 
the supposition that the above amplitudes are correct ; that is, upon the supposition 
that there is no local attraction. 
