TRANSACTIONS OF SECTION E. Wh! 
ares north of the equator, indicated that the opposite hemispheres of the globe were 
seemingly of different ellipticities. He succeeded in tracing this anomaly to an 
error in the astronomical amplitude of the arc, which had been caused by detlection 
of the plumb-line at the ends of the arc, under the influence of the attraction of 
neighbouring mountains. Thus he became aware of the necessity of placing the 
astronomical stations of the Indian ares at points where the plumb-line would not 
be liable to material deflection by the attraction of neighbouring mountain ranges. 
Shortly after his return to India Lambton died, and Everest succeeded him, and 
immediately concentrated his energies on the extension of the Great Arc northwards. 
He soon came to the conclusion that his instrumental equipment, though good for 
the time when it was procured, and amply sufficient for ordinary geographical 
purposes, was inadequate for the requirements of geodesy, and generally inferior to 
the equipments of the geodetic surveys then in progress in Europe. He therefore 
proceeded to Europe to study the procedure of the English and French surveys, 
and also to obtain a supply of new instruments of the latest and most improved 
forms. The Court of Directors of the Honourable East India Company accorded a 
most liberal assent to all his proposals, and gave him carte blanche to provide 
himself with whatever he considered desirable to satisfy all the requirements of 
science. 
Eyerest returned to India with his new instrumental equipment in 1880, a 
year that marks the transition of the character of the operations from an order of 
accuracy which was suiflicient as a basis for the graphical delineation of a 
comparatively small portion of the earth’s surface, to the higher precision and 
refinement which modern geodesists have deemed essentially necessary for the 
determination of the figure and dimensions of the earth as a whole. He imme- 
diately introduced an important modification of the general design of the principal 
triangulation, which up to that time had been thrown as a network over the country 
on either side of the Great Arc, as in the English survey and many others; but 
he abandoned this method, and, adopting that of the French survey instead, he 
devised a system of meridional chains, to be carried at intervals of about 1° apart, 
and tied together by longitudinal chains at intervals of about 5°, the whole 
forming, from its resemblance to the homely culinary utensil with which we are all 
familiar, what has been called the gridiron system in contradistinction to the net- 
work, The entire triangulation was to rest on base-lines to be measured with the 
new Colby apparatus of compensation bars and microscopes which had been con- 
structed to supersede the measuring chain the Emperor of China had rejected; the 
base-lines were to be placed at the intersections of the longitudinal chains of 
triangles with the central meridional or axial chain, and also at the further angles 
of the gridirons on each side. Latitudes were to be measured at certain of the 
stations of the central chain, with new astronomical circles in place of the old 
zenith sector, to give the required meridional arcs of amplitude. Two radical 
improvements on all previous procedure were introduced in the measurement of 
the principal angles, one affecting the observations, the other the objects observed. 
The great theodolites were manipulated in such a manner as not merely to reduce 
the effects of accidental errors by numerous repetitions in the usual way, but 
absolutely to eliminate all periodic errors of graduation by systematic changes of 
the position of the azimuthal circle relatively to the telescope, in the course of the 
complete series of measures of every angle. The objects formerly observed had 
been cairns of stones or other opaque signals; for these Everest substituted luminous 
signals, lamps by night, and, by day, heliotropes which were manipulated to reflect 
the sun’s rays through diaphragms of small aperture, in pencils appearing like 
bright stars and capable of penetrating a dense atmosphere through which distant 
opaque objects could not be seen. 
Everest’s programme of procedure furnished the guiding principles on which the 
operations were carried out during the period of half a century which intervened 
between their commencement under his superintendence and the completion of the 
principal triangulation under myself. The externai chains have necessarily been 
taken along the winding course of the frontier and coast lines instead of the direct 
and more symmetrical lines of the meridians and the parallels of latitude. The 
