TRANSACTIONS OF SECTION E. 1115 
about the same time geodesists in Europe began to recognise the advantages to 
science to be acquired by connecting the triangulations of the different nationalities 
together, and supplementing them with ares of amplitude. The ‘International 
Geodetic Association for the Measurement of Degrees in Europe’ was formed in 
consequence, and it has been, and is still, actively employed in carrying out this 
object ; in India, however, the triangulation was complete and connected through- 
out, so that only the astronomical amplitudes were wanting. They are still in 
progress, but already meridional chains, aggregating 1,840 miles im length, and 
lying to the west of the Great Arc, have been converted into meridional arcs; and 
the three longitudinal chains, from Madras to Mangalore, from Bombay to Vizaga- 
patam, and from Kurrachee vid Calcutta to Chittagong, of which the aggregate 
length is 2,600 miles, have been converted into ares of parallel. In the former the 
operations follow the meridional course of the chains of triangles; in the latter 
they follow the principal lines of the electric telegraph, which sometimes diverge 
greatly from the direction of the longitudinal chains of triangles, the two only 
intersecting at occasional points; the astronomical stations are therefore placed at 
the trigonometrical points which may happen to be nearest the telegraph lines, 
whether on the meridional or on the longitudinal chains, and their positions are 
invariably so selected as to form self-verificatory circuits which are usually of a 
triangular form, presenting three differential arcs of longitude; each of these arcs 
is measured independently as regards the astronomical work—though for the 
third arc there is usually no independent telegraph line, but only a coupling of 
the lines for the first and second arcs—and this has been proved to give such an 
excellent check on the accuracy of the operations, that it is not too much to say 
that no telegraphic longitude operations are entirely reliable which have not been 
verified in some such manner. 
Through the courtesy of Colonel Stotherd, Director-General of the Ordnance. 
Survey, I am enabled to exhibit two charts, one of the triangulation of India, the 
other of that of Europe, which have recently been enlarged to the same scale in the 
Ordnance Survey Office at Southampton for purposes of comparison. The first is 
taken from the official chart of the Indian survey, and shows the great meridional 
and longitudinal chains and Lambton’s network of principal triangles, the positions 
of the base-lines measured with the Colby apparatus, the latitude and the differential 
longitude stations, the triangular circuits of the longitudinal arcs, the stations cf the 
pendulum and the tidal operations which will be noticed presently, and the 
secondary triangulations to fix the peaks of the Himalayan and Sulimani ranges, 
and the positions of Bangkok in Siam and Kandahar in Afghanistan, the extreme 
eastern and western points yet reached. The chart of the European triangulation 
has been enlarged from one published by the International Geodetic Association of 
Europe; in it special prominence is given to the Russian meridional are, which 
extends from the Danube to the Arctic Ocean, and is 25° 20’ in length, and to the 
combined English and French meridional arc, 22° 10’ in length, which extends 
from the Balearic Island of Formentera in the Mediterranean, to Saxavord in the 
Shetland Islands. The aggregate length of the meridional arcs already completed 
in India is about equal to that of the English, French and Russian arcs combined ; 
but the longest in India is about 13° shorter than the Russian. As regards longi- 
tudinal arcs, I believe the two which were first measured in India, and were 
employed shortly afterwards by Colonel Clarke in his last investigation of the 
figure of the Earth, are the only ones which have as yet been deemed sufficiently 
accurate to be made use of in such investigations, though arcs of much greater 
length have been measured in Europe. It would be interesting, if time permitted, 
to set forth the salient points of divergence between the systems of the Indian and 
the European surveys; I will only mention that in the southern part of the 
Russian arc, for a space of about 8° from the Duna to the Dneister, a vast plain, 
covered with immense and almost impenetrable forests, presented great obstacles to 
the prosecution of the work ; the difficulty was overcome by the erection of a large 
number of lofty stations of observation, wooden scaffoldings which were 120 and 
even as much as 146 feet high, to overlook the forests. In Indian forests, as the 
Terai on the borders between British and Nepalese territories, the stations were 
