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following year a more definite scheme was called for, and
Lambton's reply is the earliest document in the records of the
Trigonometrical Branch of the Survey of India. In this letter
he points out the fallacy of the systems of survey then followed
in India, which ignored the curvature and form of the earth.
He advocates the measurement of a base-line as a 'datum';
triangulation and its correct computation; measurements to
determine the length of a degree on the meridian and per-
pendicular; and he refers to pendulum experiments and
Newton's investigations regarding the figure of the earth. He
proposes to carry out his work with a view to the requirements
of geodesy, and to follow the method adopted in the English
Ordnance Survey which had been commenced by General Roy
in 1784. Lambton's project was favourably received, and he
commenced work in 800o, the instruments available being
a steel measuring-chain of loo feet divided into forty links of
2- feet each, a 5-foot zenith sector, and a transit instrument by
Troughton. The first base of 7-43 miles was measured near
Bangalore with the chain; and for the next two years Lambton
was employed in fixing, by triangulation, a large number of
points in Mysore, to serve as a basis for the topographical
survey then in progress under Colonel Colin Mackenzie.
April io, I802, is the date of the actual commencement of the
Great Trigonometrical Survey of India, when Lambton, with
new instruments, including a 36-inch theodolite, started the
measurement of a base-line near Madras for the determination
of the length of an arc of meridian. Between I8oo and 1823
Lambton was continuously employed, practically single-handed,
on triangulation; and during that period he threw a network of
triangles, verified by several chain-measured base-lines, over the
Peninsula south of latitude I8° N., omitting the plains of
Tanjore. He also carried a chain of principal triangulation,
approximately along the meridian 78S E. from Cape Comorin
to near Ellichpur, through thirteen degrees of latitude; and
this chain, which now extends through twenty-two degrees of
latitude from Cape Comorin to the Himalayas, is known as the
Great Arc Series. Lambton's series of triangles along the
parallel 13° N., from Mangalore to Madras, was the first
attempt at a longitudinal arc. By its means he detected an
error of 40 miles in the breadth of that part of the Peninsula as
laid down by Rennell, and found great inaccuracies in the
positions of the chief towns.
In x8i8 the Trigonometrical Survey, which had up to that
date been under the Madras Government, was transferred to
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