marked object, and observe the same through the compass sights. If the same object is seen through both, and the verniers read to 90° on the limb, the adjustment is complete; if not, the correction must be made by moving the sights or changing the position of the verniers. The adjustments are all made by the maker of the instrument, and, ordinarily, need not concern the surveyor, as the instrument is very little liable to derangement. 244. Use of the Solar Compass. The declination of the sun, or its angular distance from the celestial equator, must be set off on the declination arc. The declination of the sun for apparent noon at Greenwich, England, is given from year to year in the Nautical Almanac. To determine the declination for another place and hour, allowance must be made for the difference of time arising from longitude, and for the change of declination from hour to hour. The longitude of the place can be determined with. sufficient accuracy by reference to that of given prominent places which are situated nearly on the same meridian. The difference of longitude, divided by 15, will, by changing degrees, minutes, and seconds into hours, minutes, and seconds, give the difference of time, which is usually taken to the nearest hour, as it will be sufficiently accurate. In practice, surveyors in states just east of the Mississippi allow a difference of 6 hours for longitude; S. N. 18. 7 hours for about the longitude of Santa Fé; 8 hours for California and Oregon; 5 hours for the eastern portions of the United States. Having found the hour at any place from its longitude when it is noon at Greenwich, the declination for noon at Greenwich will be the declination for the determined hour at the given place. To find the declination for the following hours of the day, add or subtract, for each succeeding hour, the difference of declination for 1 hour, as given in the almanac. Thus, let it be required to find the declination of the sun for the different hours of May 20th, 1873. W. lon. 95°. 95° 6 h. 20 m., practically 6 h. Sun's dec., Greenwich, noon ... Sun's dec., lon. 95°, 6 A. M. Add difference for 1 h. Sun's dec. 7 A. M. Sun's dec. 8 A. M. 20° 3' 14".6 20° 3′ 14′′.6 20° 3′ 45′′.63 20° 4′ 16′′.66 In like manner proceed for the remaining hours. Such a calculation should be made before beginning the work of the day. Refraction, or the bending of the sun's rays as they pass obliquely through the atmosphere, affects its declination by increasing its apparent altitude. The amount of refraction depends upon the altitude, being less as the altitude is greater. At the horizon the refraction is 35'; at the altitude of 45°, 1'; at the zenith, O. Meridional refraction, by increasing the apparent altitude of the sun, when on the meridian, increases or diminishes its apparent declination according as it is north or south of the equator. To find the amount of meridional refraction, we must first find the meridional altitude of the sun for the given latitude, which is equal to the complement of the latitude, plus or minus the declination, according as the sun is north or south of the equator. The meridional altitude of the sun being given, the tables will give the refraction. The meridional refraction, being quite small, may be disregarded in practice except when great accuracy is required, as in running great standard meridians or base lines. Incidental refraction, as affected by the hour of the day and the state of the atmosphere, can not, in practice, be determined by a precise calculation. It will about compensate for incidental refraction to keep the image of the sun square between the equinoctial lines for the middle of the day, but toward morning or evening, to run the image, which is then hazy round the edge, so that the hazy edge shall overlap one or two lines of the spaces below. To set off the latitude, find the declination of the sun for the given day at noon, and set it off on the declination arc, and clamp the arm firmly to the arc. Find in the almanac the equation of time for the given day, in order to ascertain the time when the sun will reach the meridian. About twenty minutes before noon, set up the instrument, level it carefully, fix the divided surface of the declination arc at 12 on the hour circle, and turn the instrument on its spindle till the solar lens is brought into the direction of the sun. Loosen the clamp screw of the latitude arc, raise or lower this arc with the tangent screw till the image of the sun is brought precisely between the equatorial lines, and turn the instrument so as to keep the image between the hour lines on the plate. As the sun ascends, in approaching the meridian, its image will move below the lines, and the arc must be moved to follow it. Keep the image between the two sets of lines till it begins to pass above the equatorial, which is the moment after it passes the meridian. Read off the vernier of the arc, and we have the latitude of the place which is to be set off on the latitude arc. To run lines with the solar compass.-Having adjusted the instrument and set off the declination and latitude, the surveyor places the instrument over the station, levels it carefully, clamps the plates at zero on the horizontal limb, and directs the sights north and south, approximately, by the needle. The solar lens is then turned toward the sun, and with one hand on the instrument, and the other on the revolving arm, both are moved from side to side till the image of the sun is made to appear on the silver plate, and is brought precisely within the equatorial lines, when the line of sights will indicate the true meridian. In running an east and west line, the verniers of the horizontal limb are set at 90°, and the sun's image kept between the equatorial lines. The needle is made to indicate zero on the arc of the compass box by turning the tangent screw. Lines can then be run by the needle in the temporary disappearance of the sun. The variation of the needle, which should be noted at every station, is read off to minutes on the arc along the edge of which the vernier of the needle box moves. Since the limb must be clamped at 0 when the sun's image is in position, in order that the sights may indicate the meridian, it is evident that the bearing of any line may be found by the solar compass, as well as by the compass or transit. In running long lines, allowance must be made for the curvature of the earth. Thus, in running north or south the latitude changes 1' for 92.30 ch.; and six miles, or one side of a township, requires a change of 5' 12" on the latitude arc. In running east and west lines, the sights are set at 90° on the limb, and the line run at right angles to the meridian; but this line, if sufficiently produced, would cross the equator. Hence, at the next station, a backsight is taken, and one-half the error is set off for the next foresight on the side toward the pole. The most favorable season for using the solar compass is the summer; and the most favorable time of day, between 8 and 11 A. M., and 1 and 5 P. M. A solar telescope compass is sometimes used; and, in this case, the telescope is placed at one side of the center. All error from this position of the telescope is avoided by an offset from the flag-staff. The solar compass, while indispensable in the survey of public lands, can be used, in common practice, with considerable advantage over ordinary needle instruments, since lines can be run by it without regard to the variation of the needle or local attraction, and the bearings being taken from the true meridian will remain constant for all time. |