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turning the sights to C, and it bears N. 45° 4 E. and abating three degrees, the attraction, it leaves the true bearing N. 42° 4 E. 7 chains, 28 links; then, by a back-sight, B bears from C, S. 40° W., hence at this station, the north point is attracted 24 degrees westerly ; then turning the sights to D, it bears N. 2; E. and the attraction being abated, makes it a due north course, 4 chains, 10 links; then, by a back-sight, C bears from D, S. 4° W. Hence the north point here is attracted 4 degrees westerly; then turning the sights to E, it bears S. 57° 4 E. from which take the attraction, and it leaves the true bearing, S. 53°4, E. 10 chains, 75 links; then, by a back-sight, D bears from E, N. 50 W. therefore, here the north point is attracted 34 degrees easterly; then, turning the sights to F, it bears S. 18 E. to this add the attraction 34 degrees, and it gives the true bearing, S. 21°3, E. 8 chains, 97 links; then, by a back-sight, E bears from F, N. 19°, + W. hence the north point here is attracted two degrees westerly; then, turning the sights to A, the place of beginning, it bears, N. 89 W. From this, take the attraction, and it leaves the eourse to be due west. 11 chains, 93 links.
Care must be taken to have things in good order; the needle must traverse well, the instrument set level, a small straight rod to take the sights to, and that set perpendieular, otherwise inaccuracies will arise.
The attraction, when westerly, and the next station is to the westward of the meridian, then the attraction must be added to the bearing given by the needle, for the true bearing; but if the next station be eastward of the meridian, then the attraction must be taken from the bearing given by the needle for the true bearing: so, when the attraction is easterly, and the succeeding station eastward of the meridian, then the attraction must be added to the bearing given by the needle for the true bearing; but if the next station be west of the meridian, then the attraction must be taken from the bearing given by the needle for the true bearing between the present and next stations.
These observations being strictly attended to, the bearings of a field may be truly taken, where the needle is attracted; although the station, as at H, be so remote from the land to be surveyed, as to require several stations and courses to arrive at one corner of the field.
IT is well known, that there is a Variation in the needle, from pointing due North and South ; and this variation is continually progressive. Hence it is, that the bearings of lines, taken some years ago, and the present time, do not agree; therefore, to find the difference between them, and the true bearing, at the present time.
Run the line according to the given bearing, and observe the nearest distance between the line so run, and the corner. Then—
As the length of the whole line,
ExAMPLE. Suppose a line, some years ago, bore N. 45° E. distance *
20 chains; and in running this line, by the given bearing, the corner is found 20 links to the left hand.
This rule is sufficiently exact in common practice, where the bearing and measure were both true; but in some old surveys, there are great inaccuracies : Therefore, the best way is, after running on the given bearings a convenient distance, to take the bearing and distance to the succeeding corner. Then find the bearing and distance between the corners, as taught in Division of Land, and in Intersections.
RULE 1. To the log. Secant of the latitude, rejecting the index, add the log. sine of the sun’s declination, corrected for the time and place of observation; their sum will be the log. sine of the true amplitude, to be reckoned from the east in the morning, or west in the afternoon, towards the north or south according to the declination.
2. Then if the true and magnetic amplitudes, be both north, or both south, their difference is the variation; but if one be north and the other south, their sum is the variation; and to know whether it be easterly or westerly, suppose the observer looking towards that point of the compass representing the magnetic amplitude. Then if the true amplitude be to the right hand of the magnetic amplitude, the variation is east; but if to the left hand, it is west.