theodolite :» The following important experiment has recently been tried at Brighton, in Sussex. On the new or Western Pier a good theodolite was fixed, at an elevation of 30 feet above the water, and directed to a given point on the pier at Worthing, a distance of at least ten statute miles.
A theodolite is a precision instrument used for measuring angles both horizontally and vertically. Theodolites can rotate along their horizontal axis as well as their vertical axis. Theodolites have a lot in common with transits. A transit is a surveying instrument that also takes accurate angular measurements.
Several small yachts and other vessels were sailing about between the two piers, one of which was brought to within a few yards of the Brighton Pier, and directed to sail as nearly as possible in a straight line towards the pier at Worthing; when the top of the mast, which scarcely reached the theodolite, was observed to continue below the line of sight throughout the whole distance, as shown in fig. 14–A,
representing the theodolite, and B, the pier at Worthing. From which it is concluded that the surface of the water is horizontal throughout the whole length of ten miles. Whereas, if the earth is a globe, the water between the two piers would be an arc of a circle (as shown in fig. 15), the centre of which would be 16 feet 8 inches higher than the two extremities, and the vessel starting. from A, would ascend an inclined plane, rising over 16 feet, to the summit of the arc at C, where the mast-head would stand considerably above the line of sight. From this point the vessel would gradually descend to the point B, at Worthing. As no such behaviour of the vessel was observed, the ten miles of water between the two piers must be horizontal.
