V.

An Account of the Measurement of an Arc on the Meridian on the COAST of COROMANDEL, and the Length of a Degree deduced therefrom in the Latitude 12° 32′.

BY BRIGADE MAJOR WILLIAM LAMBΤΟΝ,

IN a former Paper which I had the honour to com municate to the Asiatick Society, I gave a short sketch of an intended plan for establishing a series of connecting points commencing from the Coromandel Coast, and extending across the Peninsula; but that Paper was only meant to convey a general idea of the principles on which the work was to be conducted; a more circumstantial and scientific account, it was thought, would be more to the purpose, when I had the means of putting the plan in execution, and detailing the particulars. Since that time I have received a most complete apparatus, which has enabled me to proceed on the scale I originally proposed, and what is here offered is the beginning of that work, being the measurement of an arc on the meridian, from which is deduced the length of a degree for the latitude 12° 32' which is nearly the middle of the arc.

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The triangles here mentioned are those only, from which the arc is obtained, and the base line, the foundation to the whole, is a measured line near the Sea Coast, an account of which is here subjoined.

SECTION I. AN ACCOUNT OF THE BASE LINE.

SOME time had been taken up in examining the country best suited for this measurement, and at length a tract was found near St. Thomas's Mount, extremely well adapted for the purpose, being an entire flat, without any impediment for near eight miles, commencing at the race ground, and extending southerly. This being determined on, and the necessary preparations made, it was begun on the 10th of April, and completed on the 22nd of May,

1802.

I had expected a small transit instrument from England, for the purpose of fixing objects in the alignement, and for taking elevations and depressions at the same time; but that instrument not having arrived, I thought it unnecessary to wait, particularly as the ground was so free from ascents and descents; I therefore used the same apparatus as I had formerly done, viz. the transit circular instrument and the levelling telescope fixed on a tripod with an elevating screw in the center. In all horizontal directions, this telescope fully answers the purpose, and as there has been no deviation from the level to exceed 26' 30" excepting in one single chain, and those cases but very few, I feel entirely satisfied as to the accuracy of the whole measurement.

The chain which was made use of is the one I formerly had, and I was fortunate enough to receive another from England, made also by the late Mr. RAMSDEN, and this having been measured off by the standard in London, when the temperature was 50° by FAHRENHEIT'S thermometer, it afforded me an advantage of correcting for the effects of expansion, a circumstance in which I was by no means satistied in the former measurement. In order, therefore, to have a standard at all times to refer to, I have reserved the new chain for that purpose, and used the old one only as a measuring chain, by which means I can always determine the correction for the wear.

By referring to the annexed table, it will appear that there are only four angles of depression, and two of elevation, taken in the whole length of the base; the rest are all horizontal measurements, and many of them consist of a great number of feet before it became necessary either to sink or elevate the coffers; when that was done, great care was taken to mark the termination of the preceding measurement; and for that purpose a small tripod was used in the shape of a T, with three iron feet to run into the ground, the straight side of which T was placed in the line. Another small r was made with its top also parallel to the line, and fixed upon the large one so as to slide to the right or left, and upon that again was a long piece of brass made to slide out at right-angles to the top of the T; in the middle of this brass a mark was made, which was brought to a plumb line let fall from the arrow, and the height from the brass to the arrow was noted down; when the succeeding chain was laid, which was to commence the new level or hypothenuse, the arrow was then brought, so that a plumb line freely suspended, would coincide with the mark on the brass slider. The height of that chain above the brass was likewise taken, by comparing those two heights the elevation or depression of the new commencement was determined, and those differences noted in the seventh and eigth columns of the table. The differences of the two aggregates contained in those columns, when applied to the ascents and descents, will therefore shew how much one extremity of the base is above the other. The height of the chain at the commencement and termination of the whole was of course taken from the ground.

All the other particulars respecting this measurement are nearly the same as that in the Mysore coun

try, a full account of which has been published in a former volume of the Asiatic Researches. Some little alterations have been made in the coffers; that is, they were all of the same length, and the whole together about ninety-six feet, so as to give room for the pickets with the brass register heads. Their sides continued to the ends, and their depth on each side was the same, for the purpose of being turned every day that they might fall into a curve by their own weight and that of the chain. I also used tripods with elevating screws in the center, for supporting the coffers, making no other use of pickets than for the drawing and weight posts, and for carrying the register heads. The top of each stand on tripod was a thick circular piece of wood fixed firmly to the end of the elevating screw, and a slip of board was fastened across the circular top, screwed into the center, and allowed to turn round. When the ends of two coffers were placed on the top piece, this slip of board was admitted into the under part of each, and prevented their sliding off, a precaution that was very necessary on account of the high winds.

The point of commencement of the base was had by dropping a plummet, from the arrow of the chain suspended by a silken thread. A long but small bamboo picket had been driven into the ground till its top was level with the surface, and the cavity of the bamboo was such as just to receive the plummet, and when the first chain was in the coffers, drawn out by the weight at the opposite end, it was adjusted by the finger screw at the drawing post in such a manner that the plummet might hang suspended over the cavity of the bamboo, while the thread was applied to the arrow. This was done within the observatory tent, that the plumb line might hang freely without being disturbed by the wind. The bamboo picket was preserved with great care during the time I was observing for the latitude, and was then pro