148

HISTORY OF PHYSICAL ASTRONOMY*

ellipticity, by assuming that the zeniths of the two extreme stations weredisturbed by local attraction to the extent of 8".99. After alluding tothe inexpediency of remeasuring the arc, in consequence of the practical ,difficulty of observing the angles from Lacaille’s stations, this distin-guished officer then proceeds in the following terms :—“ It might be in- ;teresting, no doubt, to ascertain the exact latitudes of both extremes of jthe arc, by a series of triangles connecting them with the observatory now ■about to be erected in this neighbourhood, and this, which will doubtless ;be hereafter done, may in able hands furnish a new datum respecting the !attraction of mountains; but, as to the arc itself, it seems to me to be too 'small to be of any weight, even were all other objections removed, and 'the labour of correcting the old result, except for the mere curiosity of !the matter, would therefore be much better expended upon a new series ‘of triangles. Such a series, instead of terminating at Kleip Fonteyn, Imight very easily be carried through the country of the Namaquas to the inorthern boundary of the colony, which would furnish a very pretty arc 'of nearly 4" in amplitude, and, I doubt not, set for ever at rest the ano- ;malous hypothesis of the different form of the two opposite hemispheres of Jthe globe.” i

We have cited these remarks, not only because they reflect thehighest credit on the sagacity of Colonel Everest, but also on accountof the interesting confirmation which they have recently derived from thelabours of Mr. Maclear, the Astronomer Royal at the Cape of Good Hope .In the year 1837 the latter determined the latitude of Lacaille’s Obser-vatory by means of a triangulation connecting it with the Royal Obser-vatory . The result, when compared with that derived by Lacaille fromdirect observation, seemed to indicate the existence of local attraction,but the discordance was not sufficiently great to account for the wholeanomaly of Lacaille’s arc. Mr. Maclear resolved, therefore, to verify theentire operations of that astronomer by a careful remeasurement of thearc. The latitudes at the extreme stations, as determined by means ofthe zenith sector, were found to agree with the values assigned by La-caille, but the computation of the triangles had the effect of shorteningthe arc by 200 feet, and thereby reducing the anomaly in its length toabout half its previous magnitude. Suspecting that the remaining partof the error arose from the ill-conditioned character of Lacaille’s tri-angles, he chose another set of stations, and then repeated the wholeoperation; but to his great disappointment he obtained a result whichagreed almost exactly with that at which he had previously arrived. Henow resolved to measure an arc of three or four degrees, and to selectthe two extreme stations,- so as to be beyond the reach of local disturb-ance. This object he successfully effected notwithstanding many hard-ships he had to encounter in consequence of the inclemency of the weather,and the impassable nature of the country in which his operations wereconducted. The triangulation was carried southwards to Cape Point, andnorthwards as far as Kamies Berg. The distance between these two ex-tremes comprehended an arc of nearly 4° 2'; but, as Mr. Maclear sus-pected that the station at Cape Point was subject to local disturbance, hefixed the southern limit of the arc at the Royal Observatory. The dis-tance between this station and Kamies Berg included an arc of 3° 34'.Two intermediate stations were selected, one at Kleip Fonteyn, and theother to the north of that station. The final result at which he arrivedwas of the most gratifying character. He found that the length of a