FIG. 7.
A good theodolite was placed on the northern bank of the canal, midway between Welney Bridge and the Old Bedford Bridge, which are fully six miles apart, as shown in diagram, fig. 7. The line of sight from the "levelled" theodolite fell
upon the points B, B, at an altitude, making allowance for refraction, equal to that of the observer at T. Now the points B, B, being three miles from T, would have been the square of three, or nine times 8 inches, or 6 feet below the line of sight, C, T, C, as seen in the following diagram, fig. 8.
