A Treatise On The Principles And Practice Of Harbour Engineering

SURVEYING, MARINE AND SUBMARINE. 53 boat the course of the float, at any assigned moment and as rapidly as possible, he takes the readings of the two angles which the float makes with three fixed objects ranged along the shore frontage, conveniently situated as near as possible abreast of his position and preferably co- linear or nearly so.1 These angles having been plotted on a piece of tracing paper, the latter may be adjusted over a plan so that the three lines pass through the fixed points on the shore. When this con- dition is fulfilled—and there is only one Station C Station A Station B Fig. 52. position corresponding to any pair of angles,—the point may be pricked through. A station-pointer may be used in place of the tracing paper. This instru- Fig. 53. — Float used on River Avon. ment has three long flat arms, or splayed straight- edges, all radiating from a common centre. A graduated circular arc on the middle arm, with vernier indices on the side arms, enables the instru- ment to be accurately adjusted to any given com- bination of angles. This performed, the instrument is laid upon the plan so that the straight-edges pass through each of the given stations. The point of intersection of the arms is then pricked through as before. It has been remarked that for this method of locating the position of the float, two sighting in- struments are generally necessary. It is manifest that the observations must be as simultaneous as possible. Any hurry in reading the first angle prjor to adjustment for the second would lead to error. It is preferable, therefore, to fix both angles and defer the readings of either until that has been done. As a check, the angle containing the two subsidiary angles may also be read, but this in- volves the provision of a third sextant, with, pro- bably, the aid of another operator. Besides the staff or pole, other forms of float are available. On the river Avon an empty 5-gallon oil- drum has been used, sunk to almost complété immer- sion and ballasted by a basket of stone attached below it in the manner shown in fig. 53. On top of the drum is fixed a sighting mark consisting either of a semaphore, flag, or disc. 1 The method fails if all four points lie on the circumference of a cirole (fig. 52). All angles in the same or equal segments of a circle are equal. Thus the angles P]=the angles Ps.