ing S 30° W, present bearing=5°+30°=S 35° W; true bearing of O S 45° E, old bearing S 60° E, present bearing=60° -5° S 55° E; true bearing of Q-N 45° W, old bearing N 60° W, present bearing 60° -5°=N 55° W. When the north end of the needle has been moving east, then the converse of the above obtains. In Fig. 1176 true bearing of R=N 45° E, old bearing with declination 15° W N 60° E, change in declination 5° eastward, hence present bearing= 60° 5° N 55° E; true bearing of US 45° W, old S 60° W, present=60°-5° S 55° W; true bearing of T=S 45° E, old bearing S 30° E, present bearing 30°+5°=S 35° E; true bearing of V=N 45° W, old bearing N 30° W, present bearing=30°+5°N 35° W. = If the compass has no declination arc and it is desired to reduce the bearings of lines to azimuths from the north, proceed as follows: If the declination is E., with N. E. bearings add the declination; with S. E. bearings subtract the bearings a 55 FIG'S. 118. b from 180° the declination (Fig. 118 a). Using present bearings of same point as in Figs. 117, azimuth of M=35° +10°=45°; azimuth of O= (180°+10°)—55=135°; with S. W. bearings add the declination and 180°; with N. W. bearings subtract the bearings from 360°+the declination. (Fig. 118 6) azimuth of P-35°+180°+10°-225°; azimuth of Q=(360°+10°)—55°=315°. If the declination is W., with N. E. bearings subtract the declination; with S. E. bearings subtract from 180° the sum of bearings and declination. (Fig. 118 c) azimuth of R -55°-10°-45°; azimuth of T=180° — (35°+10°)=135°; with S. W. bearings add 180° -the declination; with N. W. bearings subtract from 360° the sum of bearings and declination. (Fig. 118 d) azimuth of U=55°+(180°—10°)=225°; azimuth of V=360°—(35°+10°) =315°. The following is a convenient form for the notes of a compass survey: CHAPTER IX. THE PLANE TABLE. DESCRIPTION.-The plane table (Fig. 119) consists of a drawing-board about 20x30 inches, mounted upon a tripod. It should admit of being leveled, of being turned in azimuth, and of being clamped in any position. On the drawing board is fastened a sheet of drawing paper. Upon this paper is plotted in miniature a representation of the country. Directions are not read off in degrees and minutes, but plotted directly; the instrument used being the alidade, which consists of a bevel-edged ruler, to which is attached for rough work two raised sights, as on the ordinary compass, and for the higher class of work a telescope which may be turned on a horizontal axis. The line of sight is usually, though not necessarily, parallel to the edge of the ruler. The telescope carries a delicate level and has a vertical arc for measuring angles in the vertical plane, but has no horizontal motion independent of the ruler. It also has stadia wires for reading distances. On the ruler are two levels at right angles, or one round one, for leveling the table. With the instrumental outfit is usually a declinator consisting of a compass needle, swinging about 10° each side of the zero line, encased in a long narrow box, the longer sides of which are parallel to the zero line. By means of the declinator the magnetic meridian may be marked upon the paper and then the bearing of any line determined with a protractor. Instead of a declinator, a compass, set on a heavy base with two edges parallel to the zero line, is sometimes used, and on this base are usually two level tubes at right angles. The paper used should be such as does not expand and contract differently in different directions under varying conditions of moisture. This is especially important in intersection work. If it expands and contracts uniformly in all directions, and the scale of the map is constructed upon the paper, it matters very little. To counteract unequal expansion, etc., two sheets may be mounted with cloth between them and with the grain at right angles. The paper may be attached in a variety of ways, as with thumb-tacks, with screws, or clamps for that purpose, with springs, or on rollers at each end of the table. THE ADJUSTMENT OF THE TELESCOPIC ALIDADE are similar to those of the transit, and are: (1)--For plate bubbles. (2) For line of collimation. USES OF THE PLANE TABLE.-The plane table, in some of its various forms, is the principal instrument used in "filling in" the details of a survey, when the principle points have been determined by the more precise method of triangulation. All distances are plotted as soon as determined; hence no elaborate system of notes is kept. From the primary triangulation sheet two or three points are located upon each plane-table sheet. Within these and depending upon them are then located a large number of points either by intersection, by traverse, or by both methods, forming a geometric framework upon which the sketching of the map depends. The work of making secondary and tertiary. locations by intersection is done mainly by plane table. Locations by intersection should be carried as far as practicable; that is, all points which can be well located in this manner should be so located, in order to afford the most ample control for the traversing by which the intervening areas are to be filled in, since locations by intersection are more accurate, rapid, and economic. Much misapprehension exists regarding the character and application of the plane table, from its being so little known For map-making it is the universal instrument. It is applicable to all kinds of country, to all methods of work, and to all scales. It is the most simple, direct, and economic of instruments. SETTING UP THE PLANE TABLE.-The table is set over a point by grasping the nearest two legs of tripod, and with the knee extending the third until its foot reaches the ground at the proper distance from the point, then the other two feet are set in position. To bring a point on the paper exactly over the station on the ground, use is made of a U-shaped spring, or of a frame as shown in cut. The point of one leg is placed on the point on the paper and a plumb-line is suspended from the other end. In taking up the table the nearest two legs are grasped, |