140

HISTORY OR PHYSICAL ASTRONOMY.

the English arc, and the result was generally considered by competentjudges to be equal in point of accuracy to the very best modern deter-minations of a similar character. The recent extension to which we havealluded is due to Colonel Everest, who has now prolonged the arc toKaliana, in latitude 80° 30' AB". He effected this object not by continuingthe triangulation from Ivalianpur, the northern extremity of the great arc,but by proceeding in the opposite direction from Kaliana to Kalianpur.For this purpose a base was measured in the Valley of Dhera Dhun, nearthe inferior range of the Himalayah Mountains. The extreme stationof the arc was situated at a distance of seventy miles to the south of the base,Colonel Everest justly fearing that, if a less remote locality were selected,the attraction of the Himalayah Mountains might exercise a disturbinginfluence on the celestial observations. In 1837, Colonel Everest com-pleted the triangulation as far as Kalianpur; and, as he had some doubtsrespecting the accuracy of his previous operations beyond this point, hecontinued to advance southwards, until he reached Damargida, the northernextremity of Colonel Lambton’s arc. The celestial amplitude was deter-mined by dividing the terrestrial arc into two sections at Kalianpur, andthen making simultaneous observations on the stars at the two extremitiesof each section. Thirty-six stars were observed in determining the ampli-tude of the northern section, and thirty-two in determining that of thesouthern section. In each instance half of the stars were situated to thesouth and the other half to the north of the zenith points of both ex-tremities of the arc; but the greatest distance of any star from the nearestzenith did not exceed 5°. The amplitude of the northern section wasfound to be 5° 23' 37".051, and that of the southern section, 6° 8' 65".973.This gives 11° 27' 33".024 for the amplitude of the whole arc measuredby Colonel Everest. The terrestrial length of the northern arc wasfound to be 1961157.117 feet, and that of the southern arc 2202920.196feet. Comparing these results with the amplitudes, it appears thatthe length of a degree of the meridian in mean latitude 26° 49' is363,606 feet, and that the length of a degree in mean latitude, 21° 5',is 363,187 feet. The whole of Colonel Everest’s arc, when comparedwith the English arc between Dunnose and Clifton, gives g-ig forthe ellipticity of the earth. A similar comparison with the French arcgives with the Russian arc-jA-g; and with the Swedish arc gi-g. Be-sides the base measured in the valley of Dhera Dhun, two bases of veri-fication were measured by Colonel Everest, one at the southern extremityof the whole arc, and the other near Kalianpur, the middle station. Theverification w T as effected by computing the bases at the two extremities ofthe arc from the base in the middle, and then comparing the results withthose derived from actual measurement. In this manner the length of thebase at the northern extremity was found to be 39183.273 feet by com-putation, and 39183.873 feet by actual measurement. The difference,therefore, amounted only to ^ths of a foot, or about seven inches. Again,the length of the base at the southern extremity was found to be 41578.178feet by computation, and 41578.536 feet by measurement. This gives adifference of T s,^ a 0 -ths of a foot, or a little more than four inches. The nearagreement of these results affords a strong guarantee for the accuracy ofthe whole operation. The bases were measured with an apparatus devisedby General Colby while engaged in the operations of the Irish Survey. Itconsisted essentially of tw ; o metallic bars, each ten feet long, so connected