ony 
= 
Sept. 17, 1885] 
NATURE 
487 
The trigonometrical operations fix with extreme accuracy two 
of the co-ordinates—the latitude and longitude—which define 
the positions of the principal stations ; but the third co-ordinate, 
the height, is not susceptible of being determined by such oper- 
ations with anything like the same degree of accuracy, because 
of the variations of (refraction to which rays of light passing 
through the lower strata of the atmosphere are liable, as the 
temperature of the surface of the ground changes in the course 
of the day. In the plains the apparent height of a station ten 
to twelve miles from the observer has been found to be upwards 
of 100 feet greater in the cool of the night than in the heat of 
the day, the refraction being always positive when the lower at- 
mospheric strata are chilled and laden with dew, and negative 
when they are rarefied by the heat radiated from the surface of 
the ground. At hill stations the rays of light usually pass high 
above the surface of the ground, and the diurnal variations of 
refraction are comparatively immaterial, and very good results 
are obtained by the expedient of taking the vertical observations 
between reciprocating stations at the same hour of the day, and 
as nearly as possible at the time of minimum refraction ; but in 
the plains this expedient does not usually suffice to give reliable 
results. The hill ranges of central and those of northern India 
are separated by a broad belt of plains, which embraces the 
greater portion of Sind, the Punjab, Rajputana, and the valley 
of the Ganges, and is crossed by a very large number of the 
principal chains of triansles, on the lines where the chart shows 
stretches of comparatively small triangles, which are in most 
instances of considerable length. Thus it became necessary to 
run lines of spirit levels over these plains, from sea to sea, to 
check the trigonometrical heights. The opportunity was taken 
advantage of to connect all the levels which had been executed 
for irrigation and other public works, and reduce them to a com- 
mon datum ; and eventually lines of level were carried along the 
coast and from sea to sea to connect the tidal stations. The 
aggregate length of the standard lines of level executed up to 
the present time is nearly 10,000 miles, and an extensive series 
of charts of the levels derived from other departments of the 
public service and reduced to the survey datum has already been 
published. 
The survey datun which has been adopted for all heights, 
whether deduced trigonometrically or by spirit-levelling, is the 
mean sea level as determined, either for initiation or verification, 
by tidal observations at several points on the coast lines. At 
first the observations were restricted to what was necessary for 
the requirements of the survey, and their duration was limited 
to a lunar month at each station. In 1872 more exact deter- 
minations were called for, to ascertain whether gradual changes 
in the relative level of land and sea were taking place at the 
head of the Gulf of Cutch, as had been surmised by the geo- 
logical surveyors, and observations were taken for over a year at 
three tidal stations on the coasts of the gulf, to be repeated 
hereafter when a sufficient period had elapsed to permit of a 
measurable change of level having taken place. Finally, in 
1875, the Government intimated that as ‘the great scientific 
advantages of a systematic record of tidal observations on Indian 
coasts had been frequently urged and ad nitted,” such observa- 
tions should be taken at all the principal ports and at such points 
on the coast lines as were best suited for investigations of the 
laws of the tides. In accordance: with these instructions, five years’ 
observations have been made at several points, and new stations 
are taken up as the operations at the first ones are completed. 
The initiation of the later and more elaborate operations is 
due in great measure to the recommendations of the Tidal Com- 
mittee of the British Association, of which Sir William Thom- 
son was President. The tidal observations have been treated by 
the method of harmonic analysis advocated by the Committee. 
The constants for amplitude and epoch are determined for every 
tidal component, both of long and of short periods, and with 
their aid tide-tables are now prepared and published annually for 
each of the principal ports; and further, it is with them that 
Prof. G. H. Darwin made the investigations of the effective 
rigidity of the earth, which I have already mentioned. The 
very remarkable waves which were caused by the earthquake on 
December 31, 18S1, in the Bay of Bengal, and by the notable 
volcanic eruptions in the island of Krakatoa and the Straits of 
Sunda on August 27 and 28, 1883, were registered at several of 
the tidal stations, and thus valuable evidence has been furnished 
of the velocities of both the earth-wave and the ocean-wave 
which are generated by such disturbances of the ordinarily 
quiescent condition of the earth’s crust. 
I must not close this account of the non-graphical, or more 
purely scientific, operations of the great Trigonometrical Survey 
of India without saying something of the officers who were em- 
ployed thereon, under the successive superintendence of Everest, 
Waugh, and myself. A considerable majority were military, 
from all branches of the army—the cavalry and infantry, as well 
as the corps of engineers and artillery ; the remainder were 
civilians, mostly promoted from the subordinate grades. Promi- 
nent shares in the operations were taken by Lieutenant Renny, 
Bengal Engineers, afterwards well known in this neighbourhood 
as Colonel Renny Tailyour, of Borrowfield in Forfarshire, of 
whom and his contemporary, Lieutenant Waugh, Everest, re- 
tiring, reported in terms of the highest commendation ; by 
Reginald Walker, of the Bengal Engineers, George Logan, 
George Shelverton, and Henry Beverley, all of whom fell victims 
to jungle fever; by Strange, F.R.S., of the Madras Cavalry, 
whose name is associated with the construction of the modern 
geodetic instruments of the Survey; by Jacob—afterwards 
Government Astronomer at Madras—Rivers and Haig, all of 
the Bombay Engineers; Tennant, C.I.E., F.R.S., Bengal 
Engineers, afterwards Master of the Mint in Calcutta ; Mont- 
gomerie, F.R.S., of the Bengal Engineers, whose name is best 
remembered in connection with the Trans-Himalayan geo- 
graphical operations; James Basevi, of the Bengal Engineers, 
who so sadly died of exposure while engaged on the pendulum 
operations in the higher Himalayas; Branfill, of the Bengal 
Cavalry; Thuillier, Carter, Campbell, Trotter, Heaviside, 
Rogers, Hill, and Baird, F.R.S., all engineer officers ; also 
Hennessey, C.I.E., F.R.S., M.A., Herschel, F.R.S., and 
Cole, M.A., whose names are intimately associated with the 
collateral mathematical investigations and the final reduction of 
the principal triangulation. 
The Trigonometrical Survey owes very much to the liberal 
and even generous support which it has invariably received from 
the Supreme Government, with the sanction and approval, first 
of the Directors of the East India Company, and afterwards of 
the Secretary of State for India. In times of war and financial 
embarrassment the scope of the operations has been curtailed, 
the establishments have been reduced, and some of the military 
officers sent to join the armies in the field ; but whatever the 
crisis, the operations have never been wholly suspended. Even 
during the troubles of 1857-58, following the mutiny of the 
native army, they were carried on in some parts of the country, 
though arrested in others ; and the then Viceroy, Lord Canning, 
on receiving the reports of the progress of the operations during 
that eventful period, immediately acknowledged them to the 
Surveyor-General, Colonel Waugh, in a letter from which the 
following extract is taken : 
“JT cannot resist telling you at once with how much satis- 
faction I have seen these papers. It is a pleasure to turn from 
the troubles and anxieties with which India is still beset, and to 
find that a gigantic work, of permanent peaceful usefulness, and 
one which will assuredly take the highest rank as a work of 
scientific labour and skill, has been steadily and rapidly pro- 
gressing through all the turmoil of the last two years.” 
The operations have been uninfluenced by changes of personnel 
in the administration of the Indian Empire, as Governor- 
Generals and Viceroys succeeded each other, but have met with 
uniform and consistent support and encouragement. It may 
well be doubted whether any similar undertaking, in any other 
part of the world, has been equally favoured and as munificently 
maintained. 
In conclusion I must state that I have purposely said nothing 
of the graphical operations executed in the Trigonometrical and 
other branches of the Survey of India, because they are more 
generally known, their results appear in maps which speak for 
themselves, and time would not permit of my attempting to 
describe them also. They comprise, jist, the general topo- 
graphy of all India, mostly on the standard scale of 1 inch to 
the mile ; secondly, geographical surveys and explorations of 
regions beyond the British frontier, notably such as are being 
carried on at the present time on the Russo-Afghan frontier, by 
Major Holdich and other officers of the Survey ; ¢hirdly, the so- 
called Revenue Survey of the British districts in the Bengal 
Presidency, which is simply a topographical survey on an enlarged 
scale—4 inches to the mile—showing the boundaries and areas 
of villages for fiscal requirements ; and fowrth/y, the Cadastral 
Survey of certain of the British districts in the Bengal Presi- 
dency, showing fields and the boundaries of all properties, on 
scales of 16 to 32 inches to the mile. There are also certain 
