GEOGRAPHICAL INSTRUMENTS. 235 



For the detail surveys a series of minor triangles is observed 

 by y-inch theodolites, resting on the stations of the primary 

 triangles, and with sides about a mile in length, and the boun- 

 daries and topographical details are then rilled in by the chain 

 surveyors. To the survey is added a most complete and compre- 

 hensive system of levelling, lines being run along nearly all the 

 roads, and bench marks cut at intervals of about a quarter of a 

 mile. 



It is remarkable that the commencement of the trigonome- 

 trical survey of India shoukf only have been preceded by that of 

 Great Britain, by eighteen years. The Indian Survey was begun at 

 Madras by Major Lambton in 1802, the bases being measured by 

 steel chains, and the angles taken by a 3-foot theodolite of Gary. 

 Lambton worked for twenty years, and his mantle fell upon 

 Colonel Everest, who, in 1830, re-commenced the survey with 

 Colby's compensation bars, and more perfect instruments. In 

 1841 he brought to a close the series of triangles for the 

 great arc of India,' which extends from Cape Comorin to the 

 Himalaya. The triangulation of India was continued under Sir 

 Andrew Waugh and Colonel Walker, and is now approaching 

 completion; the accuracy being equal to that attained in the 

 survey of Great Britain. In 1867 the new base line was measured 

 at Bangalore, and the difference between the measurement and the 

 length as completed through a network of triangles, from the 

 Vizagapatam base was a quarter of an inch. A new set of 

 instruments,* constructed under the superintendence of Colonel 

 Strange, has recently been forwarded to India. It consists of a 

 magnificent theodolite with a 3-foot horizontal circle, two zenith 

 sectors, two 5-foot transit instruments, two smaller ones, two 

 1 2-inch vertical circles, two galvanic chronographs for registering 

 transit observations, and three astronomical clocks. 



In India, as in Great Britain, the stations of the primary 

 triangles form the points of departure for the secondary tri- 

 * Photographs of these instruments are in the Exhibition. 



