the bearing of the line. For example, in Fig. 3, the bearing is N 65° E. The north end of the needle may be recognized by

06

E

the absence of the coil s. This coil is wound around the south half in order to balance the inclination of the needle in a vertical plane, called the dip of the needle.

Local Attraction.-The compass needle may be deflected from its natural direction by the attraction of any magnetic substance near it, such as iron ore, the rails of a railway, etc. This disturbing influence, called local attraction, is very frequently met with, and the surveyor should take special care FIG. 3 to avoid the errors to which it may give rise. When the bearing determined by a backsight does not equal that obtained by a foresight, with the letters N, S and E,W interchanged, the usual cause of the difference is local attraction. To determine whether the disturbing influence is at the end or the beginning of the line, set the compass at an intermediate point and take a sight on both points, when it will usually be found that the bearing thus obtained agrees with one of the bearings

previously found. Should this not be the case, it would tend to show that local attraction exists at both the beginning and

the end of the line, or also at the intermediate point, in which event the bearing of the line must be corrected by determining the angle by which the needle is deflected by the disturbing influences. This can be done by taking the foresight and backsight of a line formed by joining an outside point having no local attraction with the beginning or end of the line whose bearing is required.

Form for Compass Field Notes.-In Fig. 4 is shown a convenient form for keeping the notes of a compass survey. The left-hand half of the diagram represents the left-hand page of

The

The

the notebook; the right-hand half, the right-hand page. notes are supposed to apply to the field ABCDE, Fig. 5. corner, or station, A is the starting point of the survey, the courses being run from A to B, from B to C, etc. The notes read from bottom to top. Opposite the letter denoting a corner is given the bearing of the course running from that corner to the following one, in the order in which the survey was made. For instance, the bearing N 43° 20′ E horizontally opposite A denotes the bearing of the course AB. The number opposite a corner in the column of distances is the distance of this corner from the preceding one.

The right-hand page is used for remarks and sketches. When no objects are to be located along the line, as in the case from A to B, no sketch is necessary. Between B and C, a sketch is drawn showing the location of a road and mill with respect to the line BC. The line being run is usually represented by the center line on the right-hand page, unless objects are to be located at great distances on one side of the former line, in which case it is represented by a vertical line drawn near the right or the left edge of the page, as may be necessary. This is illustrated by the lines PQ and KL, which represent parts of BC and DE, respectively. A number written in the column of distances between two letters denoting corners, indicates the distance at which the point horizontally opposite it in the sketch is from the immediately preceding station or corner. Thus, the number 100, horizontally oppo

site P, indicates that the distance from B to P is 100 ft. Declination of the Needle.-The angle that the magnetic meridian or the direction of the needle is making with the true meridian is called the declination of the needle. When this declination is known, the true bearing of a line, that is, the angle that it makes with the true meridian, can be determined from its magnetic bearing by adding or subtracting the declination, as the case may require.

The declination of the needle has different values in different localities, and also varies from year to year in a given locality. The approximate declination of the needle in a given locality at a given time can be determined from charts published by the United States Coast and Geodetic Survey. They show lines passing through all points where the declination of the needle is the same (isogonic lines) and also lines passing through all points where the declination is zero (agonic lines). These charts give also the yearly variation of the isogonic lines, and may be used for obtaining approximate values of declination for dates other than those for which the chart is prepared.

TRANSIT SURVEYING

The engineers' or surveyors' transit is now used almost exclusively in surveying. This instrument is primarily intended for measuring angles in a horizontal plane, but some transits

U

D

S

FIG. 1

have also a vertical circle, or arc, for measuring angles in a vertical plane. Fig. 1 shows a transit of this kind with a vertical arc V and a level L on the telescope.

The transit generally has a magnetic needle and a graduated needle circle C, and can therefore be used as an ordinary compass. The line of sight, however, instead of being given by a pair of sights is defined by the axis of the telescope. The telescope revolves in a vertical plane on the transverse axis a, and is supported by the standards D. These are attached to the upper, or vernier, plate U. The lower plate carries a graduated circle called the horizontal limb. These plates rotate independently around the vertical axis of the instru

ment.

FIG. 2

The vernier plate rotates within and above the other, and to the former are attached two verniers v that travel along the graduated circle of the lower plate. The vernier plate can be clamped firmly to the lower plate by means of the clamp screw K, called the upper, or vernier, clamp; and by means of the upper tangent screw t it can be revolved slowly on the lower plate, moving the vernier along the divided circle, so that the instrument can easily be set at any given angle. The upper plate is attached to an accurately turned and slightly conical axis or spindle Q, Fig. 2, that extends down nearly to