they are known as magnetic storms. Their presence is generally indicated by rapid and apparently aimless fluctuations of the needle. They cause deflections of the needle that often amount to degree, and occasionally to and even 1 degree. The compass should not be used during magnetic storm, as its indications are then uncertain and unreliable. 2°18 N 33. To Change the Bearing of a Line From Magnetic to True or Vice Versa.-When the declination of the needle is known, the true bearing of a line can be very easily determined from its magnetic bearing and vice versa. In a great many cases one can picture in one's mind the relative positions of the true and the magnetic meridian, and of the line whose bearing is to be changed either from magnetic to true or from true to magnetic. Thus, if the declination of the needle is 2° east, and the magnetic bearing of a line is N 43° 00′ E, it is easy to see that the true bearing of the line is N 45° 00′ E, since the line is 43° east of the magnetic north, and the latter is 2° east of the true north. Usually, however, it is better to make a sketch, from which the change from one bearing to another can be readily made without any danger of confusion or error. 89°1 FIG. 10 Suppose, for example, that it is required to find the true bearing of a line whose magnetic bearing is S 89° 15′ W, the declination being 2° 18' east. In Fig. 10, the true and the magnetic meridian are shown by the lines SN and S'N', respectively, the magnetic north being 2° 18' east of the true north. The given line is represented by OA, making an angle of 89° 15' with OS'. It is evident, from the figure, that the angle that the line makes with the true meridian, or the angle AOS, is equal to the sum of the angles AOS and S'OS, or 89° 15′+ 2° 18' 91° 33'. Since this angle is greater than 90° and bearings are not reckoned above 90°, the true bearing is northwest and is equal to the angle NOA. The angle NOA is equal to 180° minus the angle AOS, or 180° - 91° 33′ = 88° 27'. The true bearing of the line OA is, therefore, N 88° 27′ W. 34. Declination Vernier.-In some compasses, the line passing through the zeros of the needle circle has not a fixed position coinciding permanently with the line of the sights, but can be turned about the center of the instrument through any required small angle to either side of the line of sight, the amount of the deviation being measured by a graduated arc attached to the needle circle that turns along a graduated arc or limb fixed to the main plate of the compass. By means of this device, called variously a declination vernier, declination arc, and variation arc, the line of zeros can be set at an angle with the line of the sights equal to the declination of the needle at any given place and time, so that the needle will read zero when the compass is sighted on a true north-and-south line. This makes it unnecessary to allow for the declination of the needle in reading the compass, saving both the trouble of doing it and the liability to error. EXAMPLES FOR PRACTICE 1. The declination being 3° 20' west, what is the true bearing of a line whose magnetic bearing is: (a) N 28° 55′ W? (b) S 37° 10' W? 2. When the declination is 5° 30' east, what is line whose magnetic bearing is: (a) S 12° 20′ E? 3. The magnetic bearing of a line is N 2° 30′ 4° 25' west; what is the true bearing of the line? Ans.{S 33° 50′ W (a) N 32° 15' W the true bearing of a E; the declination is 4. The declination being 5° 20′ east, what is the true bearing of a line whose magnetic bearing is N 86° 45′ E? Ans. S 87° 55′ E 5. In a northern locality, the bearing of a line as observed during the month of July, at 3 P. M., is N 48° 21' E; reduce the bearing to its daily mean value. Ans. N 48° 16' E 6. In latitude 34° north, the bearing of a line as observed at 8 A. M. on a day in October is S 12° 20′ E; what is the daily mean value of the bearing? Ans. S 12° 17′ E 7. As observed at 9 A. M. on a day in March, the bearing of a line in latitude 45° north is found to be S 14° 58' W; what is the daily mean value of the bearing? Ans. S 15° 2′ W 8. As observed at 2 P. M. on a day in August, the bearing of a line in latitude 32° north is found to be N 49° 33' W; reduce the bearing to its daily mean value. Ans. N 49° 36′ W 1 TRANSIT SURVEYING (PART 1) THE TRANSIT GENERAL DESCRIPTION 1. The Engineers' Transit.-The instrument that is now used most extensively in surveying is called the engineers', or surveyors', transit. It combines to an unusual degree the features of accuracy and convenience. It generally has a magnetic needle and a graduated needle circle, and can, therefore, be used as an ordinary compass. In the transit, however, the line of sight, instead of being given by a pair of sights, is defined by the axis of a telescope. The transit is primarily intended for measuring angles in a horizontal plane independently of the needle, but some transits have also a vertical circle or arc for measuring angles in a vertical plane. Usually, transits are so graduated that they measure angles, or "read," to the nearest minute; but there are some that read to the nearest 30, 20, or even 10 seconds; while in geodetic work, improved transits are used by which an angle can be measured to the nearest second. 2. The telescope T, Fig. 1, revolves in a vertical plane on the axis a, which is called the axis of revolution, or transverse axis, of the telescope, and is supported by the standards D. These are attached to the upper plate, or vernier plate, U, to which is also attached one of the small plate levels 1. Another plate level is attached to the COPYRIGHTED BY INTERNATIONAL TEXTBOOK COMPANY. ENTERED AT STATIONERS' HALL, LONDON § 14 |