EARTH,
600
FIGURE
two seconds. The connexion of Corsica with the continent by new measures is also contemplated. Rumania, which since 1895 has had a Geographical Institute of the Army, has not remained backward.1 Three base-lines have been measured with Brunner s apparatus : the western base, at Gar la Mare (6200 metres) ; the central base, in Bucarest (9420 metres); and the northern base, at Roman (7400 metres). The astronomical observations have been commenced for the measurement of an arc of parallel, supported by three bases .(the western, central, and an eastern one, at Slobozia), which will produce to the Black Sea the mean parallel, already measured from the Atlantic. On the other hand, the meridian arc, depending upon three bases (eastern, northern, and a Russian base near the Dniester), will help to join the Russian arc of Struve with the African arc of Gill, in long. 30° E. For their levellings the Rumanian geodesists have adopted the methods employed in the Nivellement general de la France, under the direction of M. Lallemand. It has already been reported that there does not exist any difference of mean level between the Black Sea and the Adriatic, contrary to what has hitherto been generally believed. Greece, also, is now in possession of a valuable survey, which has been effected under Colonel Hartl. In Russia the Topographical Section of the General Staff has begun great calculations, for the sake of unification of the various triangulations, executed at very different epochs. As spheroid of reference, the Besselian ellipsoid was made ' use of, which seems to suit better with the surface of the Russian Empire than that of Clarke, which had been utilized several times since 1880. In fact, General Bonsdorff, by a new calculation of the arc of Struve (long. 30° K), found the ellipticity 1/298-6, scarcely different from Bessel (1/299). On the other hand, Professor Shdanov, by the arcs of the parallels of 52° and 47°, combined with three meridian arcs which run across, found nearly the same result (l/299‘7). The Clarke ellipsoid has, on the contrary, a much stronger ellipticity (1/293-5), which results from the influence of the Indian arc.2 In the new mathematical treatment of the Russian triangulations, only the geometrical conditions of the system and the astronomical azimuths have been taken into account, leaving aside astronomical longitudes and latitudes, too much influenced by local attractions. It was therefore decided to compute the geodetical co-ordinates of the stations, starting from Dorpat (Yurief) as a point of reference. Besides the three fundamental chains (meridian arc of Struve, arcs of the parallels of 47 and 52°), four polygonal systems were at hand, formed by these chains, and meridian or parallel arcs, measured in the zone comprised between Warsaw and Saratof. The vastness of these systems prevented the application of a uniform method of simultaneous adjustment, and made it necessary to resort to partial adjustments before the final connexion of the several parts of the network. The results of the computations relative to the parallel of Central Europe (lat. 52°), that ranges over 69° of longitude, with a linear development of 4730 kilometres, from Yalentia (Ireland) to the Ural, are now known. The radius of curvature of this parallel is smaller than that corresponding to the Clarke ellipsoid; it belongs to another ellipsoid; according to Helmert, this might be an ellipsoid with the equatorial radius found by Clarke (which is conlirmed by the computation of the meridian arc of Struve), but with a much smaller ellipticity, smaller even than Bessel’s, i.e., 1/308. It is true that the parallel of 52° 1 2
See Report of General Bratiano. See Geodesy, by Colonel A. R. Clarke. Oxford, 1880.
OF
THE
presents a very sensible irregularity: the eastern, or Russian, half would agree well enough with Clarke’s ellipsoid, but the western half concords better with Bessel’s, its average curvature being greater than that of the eastern half. There lies a strong indication of the attraction of continents; this seems to be confirmed by the curvature of the Anglo-French meridian arc, which proves a little greater than that of the Russian arc of Struve, and by the fact that the polar semi-axes of the ellipses of these two arcs do not coincide; they separate by 4""5, in such a manner that Russian latitudes are too large by 4"-5, or the others too small by the same amount. The United States Coast and Geodetic Survey has published an account of the transcontinental triangulation and measurement of an arc of the parallel of 39°, which extends from Cape May (New Jersey), on the Atlantic coast, to Point Arena (California), on the Pacific coast, and embraces 48° 46' of longitude, with a linear development of about 4225 kilometres (2625 st. miles). The triangulation depends upon ten base-lines, with an aggregate length of 86 kilometres, the longest exceeding 17 kilometres in length, which have been measured with the utmost care. “ A characteristic of the triangulation is its rigidity, imparted to it by quadrilaterals and other polygons. In crossing the Rocky Mountains, many of its sides exceed one hundred miles in length, and there is one side reaching to a length of 294 kilometres, or 183 st. miles; the altitude of many of the stations is also considerable, reaching to 4300 metres, or 14,108 feet, in the case of Pike’s Peak, and to 14,421 feet at Mount Elbert. All geometrical conditions subsisting in the triangulation are satisfied by adjustment, inclusive of the required accord of the baselines, so that the same length for any given line is found, no matter from what line 6ne may start.” 3 Over or near the arc were distributed 109 latitude stations, occupied with zenith telescopes; / 3 azimuth stations; and 29 telegraphically determined longitudes. It has thus been possible to study in a very complete manner the deviations of the vertical, which in the mountainous regions sometimes amount to 25 seconds, and even to 29 seconds. According to Mr Schott, the average curvature of the surface of the geoid, along the parallel of 39 , approaches closely to that of the Clarke spheroid in the eastern part of the arc, which extends from the Atlantic to the Rocky Mountains, and to that of the Besselian spheroid in the western part, on the Pacific side, which represents f of the arc. Thus in North America as well as in Europe the curvature increases from east to west. Some preliminary combinations of this arc with other American arcs lead to spheroids approaching that of Clarke. The computations relative to another arc, the “eastern oblique arc of the United States,” are also available. It extends from Calais (Maine) in the north-east, to the Gulf of Mexico, and terminates at New Orleans (Louisiana,), in the south. Its length is 2612 kilometres (1623 st. miles), the difference of latitude 15° 1', and of longitude 22° 47'. In the main, the triangulation follows the Appalachian chain of mountains, bifurcating once, so .as to leave an oval space between the two branches. It includes among its stations Mount Washington (1920 metres) and Mount Mitchell (2038 metres). It depends upon six base-lines, and the adjustment is effected in the same manner as for the arc of the parallel. The astronomical data have been afforded by 71 latitude stations, 17 longitude stations, and 56 azimuth stations, distributed over the whole extent of the arc. For various reasons a selection had, however, 3 U.S. Coast and Geodetic Survey ; H. S. Pritchett,, superintendent The Transcontinental Triangulation and the American Arc oj Parallel. By C. A. Schott, assistant C. and G. S. Washington, 1900.
