PROTRACTOR (ALL. protractor, from Lat. protrahere, to draw forward, from pro, before. for tralicre, to drag. draw). An instrument used for measuring and laying down angles on paper. In its simplest form it consists merely of a semicircular scale of metal or transparent ma terial. The three-arm protractor used iu ma rine surveying is used to solve mechanically the 'three-point problem.' The middle arm is fixed with its reading edge at the zero of the scale; the other arms, pivoting at the centre of the in strument, are arranged to measure angles on each side of the middle arm and all carry ver niers. The method of using the protractor is as follows: Three objects (whose positions are marked on the chart or map ) are -elected. The angle between the right and centre objects and that between the centre and left are measured with sextants or similar instruments. The angles so obtained are transferred to the pro tractor, which is then laid upon the chart, and, with the edge of the centre arm always kept on the marked position of the centre object, the instrument is slipped along until the side arms fall upon the positions of the other two. The
centre of the protractor then indicates the posi tion occupied by the observer when the angles were taken, and crossed wire,. a hole in a glass ended tube. or a needle point serves to fix this position. which. if the angles are simultaneously observed, can be accurately ascertained even when the surveying boat is moving at a higher rate of speed than is consistent with obtaining correct soundings. In selecting the point of ob servation care should be taken that the angles exceed 30 I legru(s if possible, and that the point occupied is not near the circumference of the circle which passes through the points observed. If the angles are small, a slight error in them is likely to produce a large error in the result ing position; and if the point occupied is on the circle passing, through the three observed, its position is indeterminate.