TRANSACTIONS OF SECTION E. 1119 
suffice to give reliable results. The hill ranges cf 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 triangles, 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 oppor- 
tunity was taken advantage of to connect all the levels which had been executed for 
irrigation and other public works, and reduce them to a common datum; and 
eventually lines of level were carried along the coast and from sea to sea to con- 
nect 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 datum 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 determinations were called for, to ascertain whether 
adual 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 geological surveyors, and observa- 
tions 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 admitted, such observations 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 Committee of the British Association, of which 
Sir William Thomson 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 Professor 
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, 1881, in the Bay of Bengal, and by the notable 
voleanic eruptions in the island of Krakatoa and the Straits of Sunda on August 
27 and 28, 1888, 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 ordimarily 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 employed 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. Prominent 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, retiring, 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 
