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XXiy. On the Indian Arc of Meridian. By the Venerable J. H. Peatt, M.A., Archdeacon 
of Calcutta. Communicated by Professor Stokes, Sec. B.S. 
Eeceived September 3, — Bead November 22, 1860. 
It is with pleasure that I request the attention of the Eoyal Society to the present com^ 
munication, in continuation and completion of my former papers, because I think that 
the anomalies wEich the Indian Arc has appeared to present are here traced to the true,; 
causes. 
1. I will explain what those anomalies were. On completing a laborious and well- 
executed survey of the two northern portions of the Indian Arc of Meridian, between 
Kaliana (29° 30' 48") and Kalianpur (24° 7' 11"), and Kalianpur and Damargida 
(18° 3' 15"), Colonel Evekest found that their astronomical and geodetical amplitudes, 
differed considerably; in the higher arc the geodetic amplitude he found to be in excess; 
by 5"'236, in the lower of the two arcs in defect by 3"‘791*. The three stations had 
been selected with great care, and were finally chosen as being apparently free from all 
disturbing causes. Indeed, afoui’th station which had been at one time adopted, Takal 
Khera in Central India, was rejected by Colonel Evekest because a neighbouring hill- 
range was discovered on calculation to produce a defiection of about 5". Kaliana had 
been chosen nearly sixty miles from the lower hills at the foot of the Himmalaya Moun- 
tains, in the full conviction that it would be free from mountain infiuence. The surprise 
was therefore great when, on the completion of the survey of the two arcs in question, 
these two eiTors were brought to light. The first was attributed to the influence of the 
Himmalayas, but without any calculation ; but the second, with its negative sign, 
received no interpretation. At this stage I devised a method of calculating the effect of 
the Himmalayas by a direct process ; and found that the deflections produced are far 
greater than the errors which had to be explained, and the negative sign was left alto- 
gether unaccounted for. Thus the perplexity w^as increased. It next occurred to me 
that the vast Ocean to the south of India might have some influence on the plumb-line. 
On making the necessary calculations the effect of this cause was found, as the moim- 
tam attraction had been, to be far greater than had been anticipated; the negative sign 
was still unexplained, and the difficulties were not cleared up. No other cause of dis- 
turbance was apparent at the surface. But I showed by calculation that in the crust 
below one might exist sufficient to reduce the large deflections occasioned by the Moun- 
tains and the Ocean, and make them accord with the results deduced by Colonel Eveeest 
* Colonel Etebest used a mean figure of the earth somewhat different to that used in this paper, which 
13 taken from the volume of the British Survey lately published, and makes the errors somewhat less than, 
those above given. See Eveeest ‘ On the Indian Arc,’ 1847, p. clxxvi. 
MDCCCLXI. 4 K 
