TRANSACTIONS OF SECTION E. 1109 
mical observations were liable materially to exceed those of the survey, if executed. 
with fairly good instruments and moderate care. Now this was no new discovery, 
for already early in the eighteenth century the French Jesuits who were making a 
survey of China—with the hope of securing the protection of the Emperor, which 
they considered necessary to favour the progress of Christianity—had deliberately 
abandoned the astronomical method and employed triangulation instead. Writing 
in the name of the missionaries who were associated with him in the survey, Pére 
Regis enters fully into the relative advantages of the two methods, and gives the 
trigonometrical the preference, as best suited to enable the work to be executed in 
2 manner worthy the trust reposed in them by a wise prince, who judged it of the 
greatest importance to his State. ‘Thus,’ he says, ‘we flatter ourselves we have 
tollowed the surest course, and even the only one practicable in prosecuting the 
greatest geographical work that was ever performed according to the rules of 
art. 
What was true in those days is true still; points whose relative positions have 
been fixed by any triangulation of moderate accuracy present a more satisfactory 
and reliabie basis for topographical survey than points fixed astronomically. 
Though the lunar theory has been greatly developed since those days by the 
labours of eminent mathematicians, and the accuracy of the lunar tables and star 
€atalogues is much increased, absolute longitudes are still not susceptible of ready 
determination with great exactitude; moreover, all astronomical observations, 
whether of latitude or longitude, are liable to other than intrinsic errors, which 
arise from deflection of the plumb-line under the influence of local attractions, 
and which of themselves materially exceed the errors that would be generated in 
any fairly executed triangulation of a not excessive length, say not exceeding 500 
miles. 
Thus at the close of the last century Major Lambton, of the 33rd Regiment, 
drew up a projert for a general triangulation of Southern India. It was strongly 
supported by his commanding officer—Colonel Wellesley, afterwards the Duke of 
Wellington—and was readily sanctioned by the Madras Government; for a large 
accession of territory in the centre of the peninsula had been recently acquired, as 
the result of the Mysore campaign, by which free communication had been opened 
between the east and west coasts, of Coromandel and Malabar; and the proposed 
triangulation would not merely furnish a basis for new surveys, but connect 
together various isolated surveys which had already been completed or were then 
in progress. The Great Trigonometrical Survey of India owes its origin as such, 
and its simuitaneous inception as a geodetic survey, to Major Lambton, who 
pointed out that the trigonometrical stations must needs have their latitudes and 
longitudes determined for future reference just as the discarded astronomical 
stations, not however by direct observation, but by processes of calculation requiring 
a knowledge of the earth's figure and dimensions. But at that time the elements of 
the earth’s figure were not known with much exactitude, for all the best geodetic 
ares had been measured in high latitudes, the single short and somewhat question- 
able arc of Peru being the only one situated in the vicinity of the equator. Thus 
additional arcs in low latitudes, as those of India, were greatly needed and might 
be furnished by Lambton. He took care to set this forth very distinctly in the 
programme which he drew up for the consideration of the Madras Government, 
remarking that there was thus something still left as a desideratum for the science 
of geodesy, which his operations might supply, and that he would rejoice indeed 
should it come within his province ‘to make observations tending to elucidate so 
sublime a subject.’ 
Lambton commenced operations by measuring a base line and a small meridional 
are near Madras, and then, casting a set of triangles over the southern peninsula, 
he converted the triangles on the central meridian into a portion of what is 
now known as the Great Arc of India, measuring its angles with extreme care, 
and checking the triangulation by base lines measured at distances of 2 to 3 degrees 
apart in latitude, His principal instruments were a steel measuring chain, a great 
theodolite, and a zenith sector, each of which had a history of its own before 
coming into his hands, The chain and zenith sector were sent from England with 
