FIG. 12.
Although the experiments already described, and many similar ones, have been tried and often repeated, first in 1838, afterwards in 1844, in 1849, in 1856, and in 1862, the author was induced in 1870 to visit the scene of his former labours, and to make some other (one or more) experiment of so simple a character that no error of complicated instrument or process of surveying could possibly be involved. He left London (for Downham Market Station) on Tuesday morning, April 5, 1870, .and arrived at the Old Bedford Sluice Bridge, about two miles from the station, at twelve o'clock. The atmosphere was remarkably clear, and the sun was shining brightly on and against the western face of the bridge. On the right hand side of the arch a large notice-board was affixed (a table of tolls, &c., for navigating the canal). The lowest edge of this board was 6 feet 6 inches above the water, as shown at B, fig. 12.
A train of several empty turf boats had just entered the
canal from the River Ouse, and was about proceeding to Romsey, in Huntingdonshire. An arrangement was made with the "Captain" to place the shallowest boat the last in the train; on the lowest part of the stern of this boat a good telescope was fixed--the elevation being exactly 18 inches above the water. The sun was shining strongly against the white notice-board, the air was exceedingly still and clear, and the surface of the water "smooth as a molten mirror;" so that everything was extremely favourable for observation. At 1.15, p.m., the train of boats started for Welney. As the boats receded the notice-board was kept in view, and was plainly visible to, the naked eye for several miles; but through the telescope it was distinctly seen throughout the whole distance of six miles. But on reaching Welney Bridge, a very shallow boat was procured, and so fixed that the telescope was brought to within 8 inches of the surface of the water; and still the bottom of the notice-board was clearly visible. The elevation of the telescope being 8 inches, the line of sight would touch the. horizon, if convexity exists, at the distance of one statute mile;. the square of the remaining five miles, multiplied by 8 inches, gives a curvature of 16 feet 8 inches, so that the bottom of the notice-board--6 feet 6 inches above the water--should have been 10 feet 2 inches below the horizon, as shown in fig. 13--
[paragraph continues] B, the notice-board; H, the horizon; and T, the telescope.
