ATTRACTION IN THE CASE OF THE ENGLISH ARC. 
47 
fact, it is 87 per cent, of the whole attraction, although it is of smaller dimensions 
than those to the south of it. This arises from its proximity to the station, its 
distance being put down as 3 miles south of Burleigh Moor — that being the sup- 
posed distance of that station from the average line of coast on the north. . If I had 
made this 2 miles instead of 3, the attraction of B would have produced a deflection 
of 4"T80 instead of 3"T72, so considerable is the effect of the parts lying necirest to 
the attracted station. This shows the importance of an accurate survey being made 
of the neighbourhood of each of the terminal stations of the several portions of the 
arc, that the local deflections may be accurately calculated. 
25. Should it prove, on a careful survey of the neighbourhood of Burleigh Moor, 
that the deflection above deduced is correct, then the 
// 
Deflection at Clifton is . . . 0'912 to the south. 
Deflection at Arbury Hill is . 4‘579 to the north. 
Deflection at Blenheim is . . 5'902 to the north. 
Deflection at Greenwich is . 3‘976 to the north. 
Deflection at Dunnose is . . T062 to the north. 
And by making use of the Table from Captain Yolland’s volume quoted in art. 8, — 
II 
Deflection at Southampton will be . . 4 772 to the north. 
Deflection at Boniface Down will be . 3'542 to the north. 
Deflection at Week Down will be . . 3‘322 to the north. 
Deflection at Port Valley will be . . . 4‘352 to the north. 
Deflection at Black Down will be . . 5‘872 to the north. 
The coast about Black Down attains an altitude of about 800 feet, and the whole of 
England and Scotland lies north of it. The deflection, therefore, of the plumb-line at 
that place deduced above is about what might have been anticipated, viz. 5"’872 
northward, if the amount at Burleigh Moor be what ray calculation brings it out, 
viz. 3"'660. 
But it is only an accurate survey which can afford data fully satisfactory, upon 
which to base the calculation of the deflection of the plumb-line at the extremities of 
the several portions of the arc. When the true amounts of deflection are calculated, 
and the amplitudes corrected for local attraction, the process followed in art. 3 will 
bring out the actual ellipticity of the English arc ; when it will be seen whether it is 
or is not more curved than the ellipticity would indicate. 
Deep River, Cape of Good Hope, 
September 23, 1854. 
