1870.] The Surveys of India. 451 



and at a point where its measuring length was supposed to com- 

 mence, there was a brass scale, with divisions, which was fixed to 

 the head of another picket, distinct both from the draw-post and 

 from those supporting the coffers. This scale could, by means of a 

 screw, be moved backwards and forwards on the head of the post 

 till it coincided with the mark on the chain. A similar arrange- 

 ment was made at the other end, but the handle of the chain, 

 instead of being firmly attached to the weigh-post, as it was called, 

 had a rope passing over a pulley ; and to this rope was appended a 

 weight of 28 lbs. to keep the chain stretched. This arrangement 

 enabled the measurer to move his chain backwards or forwards with 

 the greatest nicety, and when satisfied that it was correctly placed, 

 to keep it there perfectly steady ; while, by means of the registers, 

 he marked the places of the two extremities of the chain. The 

 chain was then lifted by twenty coolies and carried forward, the 

 near end being adjusted to the scale which had before marked the 

 fore end. A new chain's length was then laid off in a similar 

 manner, and so on, until the base was finished. During these 

 operations tents were erected over the line, and thermometers were 

 placed in the coffers to determine the temperature of the chain ; 

 and the rate of expansion having been previously determined by 

 experiment, the necessary corrections were made for the varying 

 temperature of the measurement. The quantity of this correction 

 was • 00725 inch for every degree of Fahrenheit. 



Many of the triangles carried forward from this base line had 

 sides of from 30 to 40 miles in length. In computing their length 

 Colonel Lambton reduced the observed angles to the angles of the 

 chords, according to the method of De Lambre; and though he 

 computed the spherical excess, he did not use it in any other way 

 than as a measure of the accuracy of his observations. The chords, 

 which were the sides of the triangles, were then converted into 

 arches ; and as Colonel Lambton had contrived that the sides of the 

 four triangles which connected the stations at the south and north 

 extremities should lie very nearly in the direction of the meridian, 

 their sum, with very little reduction, gave the length of the inter- 

 cepted arch, which was thus found to be 95721 • 326 fathoms. By 

 a series of observations for the latitude, at the extremities of this 

 arch, made with a zenith sector, the amplitude of the arch was 

 found to be 1° * 53233, by which, dividing the length of the arch 

 just mentioned, Colonel Lambton obtained 60494 fathoms for the 

 degree of the meridian bisected by the parallel of 12° 32'. This, 

 till the survey was extended farther to the south, was the degree 

 nearest to the equator — excepting that in Peru, almost under it — 

 which had yet been measured. The next object was to measure a 

 degree perpendicular to the meridian, in the same latitude. This 

 degree was accordingly derived from a distance of more than 55 



