and subsequent triangles. These triangles near the coast were unavoidably confined and oblique; but their sides are generally deduced from larger and more regular triangles, expanding over the interior of the country. The annexed figure exhibits the most interesting portion of this memorable survey, and represents the various combination of triangles. Attached to it is a scale of English miles. A Frant Church. K Folkstone Turnpike.
. . In this register, each angle in the successive triangles is, for the sake of conciseness, marked by the single letter affixed to it, and the computed length of its opposite side in feet ranges in the same line. The addition of an asterisk denotes that an angle was not actually observed, but only deduced from calculation. The oblique triangles ABC and ABE have their sides BC and BE derived from other larger triangles, which were nearly equiangular. The triangles ELN and ENP had their angles discovered from conjoined observations. In general the several angles, as affected by the spherical excess, were corrected for computation by a sort of tentative process. It results from a train of calculations, that Dover Castle lies south 67° 44' 34m east, and at the distance of 328231 feet or 62.165 miles, from Greenwich Observatory. On their part, the French astronomers, under the direction of Cassini, carried forward the trigonometrical operations from Dunkirk to Paris; employing Borda’s repeating circle, an instrument much smaller and less perfect than Ramsden’s theodolite, but formed on a principle which always procures the observer a near compensation of errors. From a comparison of the whole, it follows, that the meridian of the Observatory of Paris lies 2° 19' 51" east from that of Greenwich, differing only nine seconds in defect from what the late Dr Maskelyne had previously determined from combined astronomical observations. The success with which that great survey was attended, gave occasion both in France and England to still more extensive projects. The National Assembly, amidst other essential improvements which it meditated, having resolved to adopt a general and consistent system of measures, the length of a degree of the meridian at the middle point between the pole and the equator was proposed as a permanent basis. But to secure greater accuracy in determining the standard, it had been decided to prolong the observations on both sides of the mean latitude, and trace a chain of triangles over the whole extent from Dunkirk to Barcelona. This bold plan was executed in the course of the years 1792, 1793, 1794 and 1795, with equal sagacity and resolution, by MM. Delambre and Mechain, who, during all the horrors of revolutionary com a motion, yet pressed forward their operations in spite of obstacles and dangers of the most sickening kind. After the various triangles, amounting in total to 115, had been observed, they were connected, in the neighbourhood of Paris, with a base of more than seven miles in length, and measuring, at the temperature of 16#8 on the centigrade scale, or 61+” by Fahrenheit, 6075.9 toises from Melun to Lieursaint. A base of verifieation was likewise traced near the southern extremity of the line of survey, extending 6006.25 toises along the road from Perpignan to Narbonne. This base appeared not to differ one foot from the calculation founded on the other, though separated by a distance of 400 miles, a convincing proof of the accuracy with which the observations had been made. A specimen of the French triangulation is given in the figure below, where the vertical line represents the meridian of Dunkirk, with the distances expressed by intervals of 10,000 toises, Fr-50 - as: A St Martin du Tértre. H-30 F Montlheri. L Forêt. GH The primary base. |