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on without any basis of operation, and afterwards compiled into 
a general map in which the positions of a few of the chief 
towns in each province were determined by astronomical ob- 
servations. Such determinations are always of questionable 
accuracy as the basis of an exact survey, even in the present 
state of science; and a century ago the tables of the places 
of the heavenly bodies were far less accurate than they are 
now, and frequently caused gross errors of position. Towards 
the close of the last century Major Lambton projected a 
“Mathematical and Geographical Survey” of southern India 
to determine by triangulation from measured base lines the 
positions of a number of permanent geographical marks to be 
afterwards the basis of a general survey of the Peninsula; but 
he pointed out that before the latitudes and longitudes of these 
marks could be correctly computed from the data furnished by 
the triangulation, the figure of the earth should be known 
with accuracy, and he suggested that his operations should be 
carried out in such a manner as to answer the requirements 
of a geodetic as well as of a geographical survey. His proposals 
were approved of by the Government, and the operations which 
he carried out were the commencement of the Great Trigono- 
metrical Survey of India. 
Col. Walker shewed that the combination of objects in 
a triangulation which is intended to serve the purposes of a 
geodetic as well as of a geographical survey necessarily intro- 
duces a very high order of accuracy into the operations, and 
that the Indian survey has derived much advantage from this 
circumstance. He described the nature of the operations of 
the survey, the manner in which they are conducted, and the 
instruments which are respectively used in making the linear 
and the angular measurements. The probable errors of the 
base lines are shewn to be = + 2°6 millionth parts of the length 
measured, corresponding to a probable error of 108 feet in 
the length of the polar axis of the earth. The probable errors 
