FIG. 16.
The sea horizon, to whatever distance it extends to the right and left of an observer on land, always appears as a perfectly straight line, as represented by H, H, in fig. 16. Not only does
it appear to be straight as far as it extends, but it may be proved to be so by the following simple experiment. At any altitude above the sea-level, fix a long board--say from 6 to 12 or more feet in length--edgewise upon tripods, as shown in fig. 17. Let
the upper edge be smooth, and perfectly levelled. On placing the eye behind and about the centre of the board B, B, and looking over it towards the sea, the distant horizon will be observed to run perfectly parallel with its upper edge. If the eye be now directed in an angular direction to the left and to the right,
there will be no difficulty in observing a length of ten to twenty miles, according to the altitude of the position; and this whole distance of twenty miles of sea horizon will be seen as a perfectly straight line. This would be impossible if the earth were a globe, and the water of the sea convex. Ten miles on each side would give a curvature of 66 feet (102 x 8 = 66 feet 8 inches), and instead of the horizon touching the board along its whole length, it would be seen to gradually decline from the centre C, and to be over 66 feet below the two extremities B, B, as shown in fig. 18. Any vessel approaching from the left would be seen to
ascend the inclined plane H, B, C, and on passing the centre would descend from C towards the curvating horizon at H. Such a phenomenon is never observed, and it may be fairly concluded that such convexity or curvature does not exist.
