| Sept. 17, 1885] 
NATURE 
485 
yet attempted in any other survey. But the happy result of all 
this labour was that the final corrections of the angles were for 
the most part very minute, less than the theoretical probable 
errors of the angles, and thus fairly applicable without taking 
any liberties with the facts of observation. If the attribute of 
beauty may ever be bestowed on such things as small numerical 
quantities, it may surely be accorded to these notable results of 
very laborious calculations, which, while in themselves so small, 
were so admirably effective in introducing harmony and precision 
throughout the entire triangulation. 
If now we turn once more to what Lambton calls ‘‘ the sub- 
lime science of geodesy,” which was held in such high regard by 
both him and Everest, we shall find that the great meridional 
are between Cape Comorin and the Himalayas, on which they 
laboured with so much energy and devotion, is not the only con- 
tribution to that science to which the Iniian triangulatlon is 
subservient, but every chain of triangles—meridional, longi- 
tudinal, or oblique—may be made to throw light either on 
geodesy, the science of the figure of the earth, or on geognosy, 
the science of the earth’s interior structure, when combined with 
corresponding astronomical arcs of amplitude. Thus each of the 
several meridional chains of triangles may be utilised in this 
way, as their prototype has been, by having latitude observations 
taken at certain of their stations to give meridional ares ; and the 
several longitudinal chains of triangles may also be utilised—in 
combination with the main lines of telegraph—by electro- 
telegraphic determinations of differential longitudes to give ares 
of parallel. When the stations of the triangulation which are 
resorted to for the astronomical observations are situated in 
localities where the normal to the surface coincides fairly with 
the corresponding normal to the earth’s figure, the result is 
valuable as a contribution to geodesy ; when the normal to the 
Surface is sensibly deflected by local attraction, the result gives a 
measure of the deflection which is valuable as a contribution to 
geognosy. 
Having regard to these circumstances, I moved the Govern- 
ment to supply the Trigonometrical Survey with the necessary 
instruments for the measurement of the supplemental astronom- 
ical ares ; and as officers became available on the gradual com- 
pletion of the successive chains of triangles, [ employed some of 
them in the required determinations of latitude and differential 
longitude. It so happened that about the same time geodesists 
in Europe began to recognise the advantages to science to be 
acquired by connecting the triangulations of the different nation- 
alities together, and supplementing them with arcs 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 
throughout, so that only the astronomical amplitudes were 
wanting. They are still in progress, but already meridional 
chains, aggregating 1,840 miles in length, and lying to the west 
of the Great Are, have been converted into meridional ares; and 
the three longitudinal chains, from Madras to Mangalore, from 
Bombay to Vizagapatam, and from Kurrachee va Calcutta to 
Chittagong, of which the aggregate length is 2,600 miles, have 
been converted into arcs of parallel. In the former the opera- 
tions 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 longi- 
tudinal 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 
a 
shows the great meridional and longitudinal chains and Lamb- 
ton’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 longi- 
tudinal arcs, the stations of 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 arc, which 
extends from the Danube to the Arctic Ocean, and is 25° 20’ in 
length, and to the combined English and French meridional are, 
22° to’ 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 
1}° shorter than the Russian. As regards longitudinal ares, I 
believe the two which were first measured in India, and were 
employed shortly afterwards by Colonel Clarke in his last investi- 
gation 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 per- 
mitted, 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 are, 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 rarely raised to a greater height 
than 30 feet, or just sufficient to overtop the curvature, and all 
trees and other obstacles were cleared away on the lines between 
them ; this was found the most expeditious and economical 
process. The stations were very substantial, with a central 
masonry pillar, for the support of a great theodolite, which was 
isolated from the surrounding platform for the support of the 
observer. The lofty Russian scaffoldings only sufficed for small 
theodolites, and they were so liable to shake and vibration, that 
the theodolites had to be fitted with two telescopes to be pointed 
simultaneously by two observers at the pair of stations, the 
angle between which was being measured. 
All the modern geodetic data of the:Indian survey that were 
available up to the year 1880 were utilised by Colonel A. R. 
Clarke, C.B., of the Ordnance Survey, in the last of the very 
valuable investigations of the Figure of the Earth which he has 
undertaken from time to time. It will be obvious that new data 
tend to modify in some degree the conclusions derived from 
previous data, for the figure of so large a globe as our earth is 
not to be exactly determined from measurements carried over a 
few narrow belts of its superficies. Thus thirty years ago it was 
inferred that the equator was sensibly elliptic—and not circular, 
as had been generally assumed—with its major axis in longitude 
15° 34 east of Greenwich ; but later investigations indicate a far 
smaller ellipticity, and place the major axis in west longitude 
8° 15’. More significant evidence of the influence of new facts 
of observation in modifying previous conclusions is furnished by 
the French national standard of length, the metre, which was 
fixed at the ten-millionth part of the length of the earth’s 
meridional quadrant, as deduced from the best geodetic data 
available up to the end of the last century ; but it is now found 
to be nearly s;4;5th part less than the magnitude which it is 
supposed to represent, the difference being about a hundred 
times greater than what would now be considered an allowable 
error in an important national standard of measure. 
The Indian survey has also made valuable contributions to 
geodesy and geognosy in an elaborate series of pendulum observ- 
ations for determining variations of gravity, which throws light 
both on the grand variation from the poles to the equator that 
governs the ellipticity, and on the local and irregular variations 
depending on the constitution of the interior of the earth’s crust. 
They were commenced in 1865 by Captain J. P. Basevi, on the 
recommendation of General Sabine and the Council of the Royal 
Society, with two pendulums, one of which the General had 
