484 
NATURE 
[ Sept. 17, 1885 
either to raise the stations to a sufficient height to overlook all 
surrounding obstacles, or to render them mutually visible by 
clearing the lines between them ; and both these processes are 
more or less tedious and costly. There are many tracts of forest 
and jungle which greatly impeded the operations, not merely 
because of the physical difficulties they presented, but because 
they teemed with malaria, and were very deadly during the 
greater portion of the year, and more particularly immediately 
after the rainy seasons, when the atmosphere is usually clearest 
and most favourable for distant observations. At first tracts of 
forest, covering extensive plains, were considered impracticable ; 
thus Lambton carried his network over the open country, and 
stopped it whenever it reached a great plain covered with forest 
and devoid of hills; but Everest’s system would not permit of 
any break of continuity, nor the abandonment of any chain 
which was required to complete a gridiren ; it has been carried 
out in all its integrity, often with much sacrifice of life, but 
never with any shrinking on the part of the survey officers from 
carrying out what it had become a point of honour with them 
to accomplish, and the accomplishment of which the Govern- 
ment had come to regard as a matter of course. We have 
already seen how the progress of Everest’s first chain of triangles 
was suddenly arrested because he and all his people were struck 
down by malaria in the pestilential regions of the Godavery 
basin. That chain remained untouched for fifty years ; it was 
then resumed and completed, but with the loss of the executive 
officer, Mr. George Shelverton, who succumbed when he had 
not yet reached, but was within sight of, the east coast line, the 
goal towards which his labours were directed. Many regions, 
as the ba-in of the Mahanaddi, the valley of Assam, the hill 
ranges of Tipperah, Chittagong, Arracan, and Burma, and 
those to the east of Moulmein and Tennasserim, which form the 
boundary between the British and the Siamese territories, are 
covered with dense forest, up to the summits of the peaks which 
had to be adopted as the sites of the survey stations. Asa rule 
the reaks were far from the nearest habitation, and they could 
not be reached until pathways to them had been cut through 
forests tangled with a dense undergrowth of tropical jungle ; 
not unfrequently large areas had to be cleared on the summits to 
open out the view of the surrounding country. Here the physical 
difficulties to be overcome were very considerable, and they were 
increased by the necessity that arose, in almost every instance, 
of importing labourers from a great distance to perform the 
necessary clearances. But the broad belt of forest tract known 
as the Terai, which is situated in the plains at the feet of the 
Nepalese Himalayas, was the most formidable region of all, 
because the climate was very deadly for a great portion of the 
year, and more particularly during the season when the atmo- 
sphere was most favourable for the observations, though the 
physical difficulties were not so great as in the hill tracts just 
mentioned, and labour was more easily procurable. Lying on 
the British frontier, at the northern extremities of no less than 
ten of the meridional chains of triangles, it had necessarily to 
be operated in to some extent,,and Everest wished to carry the 
several chains across it, on to the outer Himalayan range, and 
then to connect them together by a longitudinal chain running 
along the range from east to west, completing the gridiron in 
this quarter. But the range was a portion of the Nepalese 
territories, and all Europeans—excepting those attached to the 
British embassy at Khatmandu—were debarred from entering 
any part of Nepal, by treaty with the British Government. 
Everest hoped that the rulers of Nepal might make an exception 
in his favour for the prosecution of a scientific survey; and 
when he found they would not, he urged the Government to 
compel them to give his surveyors access, at least, to their out- 
lying hills ; but he urged in vain, for the Government would 
not run the risk of embarking in a war with Nepal for purely 
scientific purposes. Thus the connecting chain of triangles— 
now known as the N.E. Longitudinal Series—had to be carried 
through the whole length of the Terai, a distance of about 
509 miles, which involved the construction of over 100 towers— 
raised to a height of about 30 feet to overlook the earth’s 
curvature—and the clearance of about 2,000 miles of line through 
forest and jungle to render the towers mutually visible. It 
required no small courage on Everest’s part to plunge his 
surveyors into this region ; he endeavoured to minimise the risks 
as much as possible by taking up the longitudinal chain in 
sections, bit by bit, on the completion of the successive meridional 
chains, and thus apportioning it between several survey parties, 
each operating in the Terai for a short time, instead of assigning 
it to a sinzle party to execute continuously from end to end, as 
all the other chains of triangles. But notwithstanding these 
precautions, the peril was great, and the mortality among both 
officers and men was very considerable ; greater than in many 
a famous battle, says Mr. Clements Markham, in an eloquent 
passage in his Memoir of the Indian Suryeys, in which he 
claims for the surveyors who were employed on these operations 
—with no hope of reward other than the favourable notice of 
their immediate chief and colleagues—merit for more perilous 
and honourable achievement than much of the military service 
which is plentifully rewarded by the praises of men and prizes of 
all kinds. 
Everest retired in 1843, and was succeeded by Waugh, who 
applied himself energetically to the completion of the several 
chains of triangles exterior to the Great Arc, for which he 
obtained a substantial addition to the existing equipment of 
great theodolites. It was under him that the formidable 
longitudinal series through the Terai, which had been begun 
by Everest, was chiefly carried out. He personally initiated the 
determination of the positions and heights of the principal snow 
peaks of the Himalayan ranges; and he did much for the 
advancement of the general topography of India, which had 
somewhat languished under his predecessor, who had devoted 
himself chiefly to the geodetic operations. He retired in 1861, 
and I succeeded to the charge of the Great Trigonometrical 
Survey. The last chain of the principal triangulation was 
completed in 1882, shortly before my own retirement. 
Of the general character of the operations, it may be asserted 
without hesitation that a degree of accuracy and precision has 
been attained which has been reached by few and surpassed by 
none of the great national surveys carried out in other parts of 
the world, and which leaves nothing to be desired even for 
the requirements of geodesy; a very considerable majority of 
the principal angles have been measured with the great 24-inch 
and 36-inch theodolite, and their theoretical probable error 
averages about a quarter of a second ; of the linear measure- 
ments the probable error, so far as calculable, may be taken as 
not exceeding the two-millionth part of any measured length. 
And as regards the extent of the trianzulation, if we ignore 
the primary network in Southern India, and all secondary 
triangulation, however valuable for geozraphical purposes, we 
still have a number of principal chains — meridional, 
longitudinal, and oblique—of which the aggregate length is 
17,300 miles, which contain 9,230 first-class angles all ob- 
served, and rest on eleven base-lines measured with the 
Colby apparatus of compensation bars and microscopes. 
This prodigious amount of field-work furnishes an enormous 
mass of interdependent angular and linear measures ; and eaeh of 
these is fallible in some degree, for, great as was the accuracy 
and care with which they had severally been executed, perfect 
accuracy of measurement is as yet beyond human achievement ; 
thus every circuit of triangles, every chain closing on a base-line, 
and even every single triangle, presented discrepancies the mag- 
nitude of whick was greater or less according as derived from a 
combination of many, or only of a few, of the fallible facts of 
observation. Thus, when the field operations were approaching 
their termination, the question arose as to how these facts were 
to be harmonised and rendered consistent throughout, which was 
a very serious matter considering their great number. The strict” 
application of mathematical theory to a problem of this nature 
requires the adjustment to be effected by the application of a 
correction to every fact of observation, not arbitrarily, but in 
such a manner as to give it its proper weight, neither more nor 
less, in the final investigation, and in this the whole of the facts 
must be treated simultaneously. That would lave involved the 
simultaneous solution of upwards of 4,000 equations between 
9,230 unknown quantities, by what is called the method of 
minimum squares, and I need scarcely say that it is practically 
impossible to solve such a number of equations between so many 
unknown quantities by any method at all. Thus a compromise 
had to be made between the theoretically desirable and the 
practically possible. It would be out of place here to attempt 
to describe the method of treatment which was eventually 
adopted, after much thought and deliberation ; I will merely 
say that the bulk of the triangulation was divided into five 
sections, each of which was treated in succession with as close 
approximation to the mathematically rigorous method as was 
practically possible ; but even then the mass of simultaneous in- 
terdependent calculation to be performed in each instance was 
enormous, I believe greatly exceeding anything of the kind as 
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