THE TRIGONOMETRICAL SURVEY
By F. C. Danvers

THE surveys of India may be divided into two classes - viz. the Great Trigonometrical, and the Geological. In connection with the former, other minor operations are undertaken under the title of topographical and revenue surveys, to which we shall refer more particularly in due course.

The idea of a great trigonometrical survey of a country, to be undertaken by the Government of that country, was first conceived by General Watson, at the suppression of the rising in Scotland in 1745. The execution of it was committed to General Ray, and was originally intended to extend no farther than the disaffected districts of the Highlands. The design, however, was subsequently enlarged, and the grand trigonometrical survey of Great Britain and Ireland was projected. Perhaps a more important survey, in some respects, than the British one was that undertaken by the French nation at the period of the Revolution. About that date' the philosophers of France undertook to introduce a great reformation in regard to all those habits and usages of men which have reference to numbers, and everything-lengths, areas, moneys, weights, periods of time, arcs of circles-was to be numbered by tens, hundreds, thousands, &c. The question then came to be, What should be adopted as the basis of this standard, which was designed not only for France, but for the world? This question having been brought to the attention of the Constituent Assembly, it was proposed by M. de Talleyrand, and decreed accordingly, that the Parliament of England should be requested to concur with the National Assembly in fixing a natural unit of weights and measures; that under the auspices of the two nations, an equal number of Commissioners from the Academy of Sciences and the Royal Society of London might unite in order to determine the length of the pendulum which vibrates seconds in the latitude of 45° (as proposed originally by Huyghens), or in any other latitude that might be thought preferable, and to deduce from them an invariable standard of measures and of weights. The Commission named by the Academy had under their consideration three different units, namely, the length of the pendulum, the quadrant of the meridian, and the quadrant of the equator. The length of a quadrant of the meridian having been determined on, the measurement of an arc was entrusted to MM. Mechain and De Lambre, who began their labours in 1792, and thus commenced the trigonometrical survey of France.

The origin of the Great Trigonometrical Survey of India was not unlike that of the first Scottish Survey. After the successful termination of the war with Tippoo Saib, at the close of the last century, Captain Lambton (who had previously served as a surveyor in America, and who joined Her Majesty's 33rd Regiment at Calcutta in the year 1797) brought forward his plan of a geographical survey of part of the territory that had been conquered, and he proposed to throw a series of triangles across from Madras to the opposite coast, for the purpose of determining the breadth of the peninsula in that latitude, and of fixing the latitudes and longitudes of a great many important places, which were believed to be very erroneously determined in the survey previously executed by Colonel MacKenzie. Captain Lambton first submitted his plan to Colonel Wellesley, in whose regiment he had formerly served, who at once sent up the proposal to Government supported by his strong recommendation. Lord Clive was at that time Governor of Madras, and warmly approved of the undertaking, and it was accordingly sanctioned by Government.

The first base line measured by Colonel Lambton WM on the Table-land of Mysore, near to Bangalore. The chain used by him was one of blistered steel, constructed by Ramsden, and precisely similar in every respect to the one used by General Roy in measuring his base of verification on Rumney Marsh. It consisted of forty links of 2 1/8 feet each, measuring in the whole 100 feet, at a temperature of 62°, and fitted with two brass register heads, with a scale of 6 inches to each. This chain, it appears, had originally been sent with Lord Macartney's embassy as a present to the Emperor of China, and having been refused by him, it was made over by his Lordship to the astronomer, Dr. Dinwiddie, from whom it was purchased. The measurement of this base line was commenced on the 14th October 1800, and completed on the 10th December following. Its total length was 7.4321 miles.

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