HIMALAYAS ON THE PLUMB-LINE IN INDIA. 
71 
29. These distances should be laid down and the circles drawn on a map or globe ; 
and nothing remains to be done, but to ascertain the average heights of the masses 
standing on the compartments thus drawn. 
If the surface of any of these masses is very irrfegular, several vertical sections 
should be taken in directions most favourable for giving a fair average. One con- 
venient method of using such sections is, after laying them down on a scale on good 
paper, to cut them out, weigh them, and compare the weight with that of a portion 
of the same kind of paper of known dimensions on the same scale. The resulting 
number divided by the aggregate length of all the sections will give the average 
height. It may be convenient to use different scales for the vertical and hori- 
zontal measures : this may be done if it be carefully attended to in carrying out this 
method. 
30. It will be evident that mountain ranges will assume a less importance in this 
calculation as they are more distant from A, since they will stand on a larger com- 
partment ; and therefore when in imagination levelled down to cover the whole 
compartment and give the average height, they will stand at a much less altitude. 
It is for this reason that a knowledge of extensive table-lands of considerable ele- 
vation, and of the elevated channels of rivers, is of far more importance in this 
calculation — especially in the remoter parts — than of mountain peaks and mountain 
ridges. 
II. Approximation to the amount of mountain attraction at the two extremities and the 
middle station of the Great Indian Arc of the Meridian between latitudes 18° 3' 15" 
and 29° 30' 48". 
31. The complete application of the method I have developed requires a full 
survey of the earth’s superficies. In the absence, however, of sufficiently accurate 
and extensive information to make an exact calculation, I propose now to use such 
data as I have been able to gather — chiefly from books on geography and Hum- 
boldt’s works, as well as the published Maps of the Indian Survey — to obtain an 
approximation to the amount of attraction on the plumb-line on the Indian arc. 
I regret that absence from India — which is the occasion of my finding leisure to 
draw up this paper — prevents my consulting Humboldt’s Map of Central Asia 
(published in 1842), although extracts from his ‘Aspects of Nature’ will in part 
supply the want. 
32. Fig. 4 represents an outline of the continent of Asia. A, B, C are the northern, 
middle, and southern stations of the Great Arc I am about to consider. These stations 
are Kaliana in latitude 29° 30' 48", Kalianpur in 24° 7 ' 1 1”? and Damargida in 
18° 3' 15". The longitude of the arc is about 77° 42', 
The polygonal figure DEFGHIJKLD, which for the convenience of a name I 
shall call the Enclosed Space, marks out the boundary of an irregular mass, which is 
the only part of the earth’s surface that appears to have a sensible effect on the plumb- 
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