It will be necessary to know the times of the greatest elongation. These times are given in the following tables, for the 1st, 11th, and 21st of each month of the year 1880, which will answer the purpose for the rest of the century, since the change of time is very slow, being only about 16 minutes in 50 years. About half an hour before the greatest eastern or western elongation, place the transit in a convenient position, and level it carefully. Paste white paper on a board about one foot square, and perforate the board through the center with a twoinch auger, and, on the lower edge, fix some contrivance for holding a candle. 11th day. 5h.52m. A.M. 11h. 44m. P.M. 9h. 03m. P.M. Let this board be fixed to a vertical staff, so as to slide freely up and down, and let it be placed about one foot in front of the telescope, so that the light reflected from the paper will render the spider lines visible. Slide the board up or down the staff till the North star is visible through the telescope and orifice in the board, and bring the vertical spider line in range with the star. As the star approaches its greatest elongation, move the telescope by a tangent screw, so as to keep the vertical line in range with the star. When the star reaches its greatest elongation, it will appear, for some time, to coincide with the spider line, and then leave it in the opposite direction. Clamp the horizontal limb, and turn the telescope down till it is horizontal. Let now a staff, with a light on its upper end, be carried ten or fifteen rods distant, toward the star, and placed so as to range, when vertical, with the vertical spider line of the telescope. Drive a stake at the foot of the staff, and another directly under the instrument, then will the line determined by the stakes make an angle with the true meridian, equal to the azimuth of the North star. The true meridian will lie west or east of the C B line of stakes, north of the telescope, according as the elongation was east or west, and may readily be located by the instrument. The location of the meridian can be verified thus: Let AB be the line of the stakes produced to a considerable distance, say from 20 to 40 chains, A the azimuth angle, AC the true meridian, and BC perpendicular to AB. BC can be found from the formula, Then laying off BC on the ground, and driving a stake at C, the stakes A and C will trace the true meridian. Having found the true meridian, the variation of the needle can be readily determined by turning the telescope or the sights of the compass in the direction AC. Without finding the true meridian, the bearing of AB being equal to the known azimuth of the North star at its greatest elongation, the variation of the needle can be found by directing the telescope or the sights of the compass in the direction AB. The following method may be resorted to by the surveyor who does not possess an instrument with a telescope. Fix a plank, firmly level, east and west, about three feet above the ground; then take a board about six inches square, and having detached one of the compass sights, fix it to the board, at right angles with its upper edge. Drive a nail obliquely a little way into the board, so that it can be tacked to the plank. About fifteen feet north of the plank suspend a plumb line, from the top of an inclined stake of such height that the North star, when seen through the sight while the board rests on the plank, will appear about one foot below the upper end of the plumb line. Suspend the plumb in a vessel of water to prevent the line from vibrating, and let an assistant hold a light near it, so that it can be seen through the sight. About half an hour before the time of the greatest elongation of the North star, place the board on the plank, and slide so that the star and plumb line shall range when seen through the sight. As the star approaches its greatest elongation, move the board along the plank in the opposite direction, so as to keep the range. When the star reaches its greatest elongation, it will appear to keep the range for several minutes, then it will move slowly in the opposite direction. Tack the board to the plank, taking care not to change its position. Then let a staff with a light on its top be placed about ten rods farther to the north, so as to range, when vertical, through the sight, with the plumb line. Drive a stake at the foot of the staff, and one directly under the plumb line, then will the line of the stakes make, with the meridian, an angle equal to the azimuth of the North star at its greatest elongation. The true meridian, and the variation of the compass, can then be found as above. FIELD OPERATIONS. 275. Finding Corners. In searching for a corner, first seek for the monument, whether tree, post, stake, or stone, as given and witnessed in the original field notes, which, if found, must be considered decisive in establishing the corner. If no monument can be found, the corner can often be found by indirect methods, of which the following are the most available: Thus, if a monument can be found at each of the corners A, C, D, but not at B, find the corners E and F, at each of which set up a flag- A staff or high pole, and send the flag-man as near to B as possible, and let him stand facing D, so that he can see signals made both at A and C. D F E The observer at A can, by waving his hand, bring the flag-man in the line AE, and the observer at C can bring him in the line CF, and being in both lines, AE and CF, at the same time, he will be at their intersection B, the corner required. If the corner E can be found, but not F, measure AB the required distance in the line AE. If the distance AB is not known, but it is simply known that AB is equal to DC, first measure DC. If neither E nor F can be found, run AB parallel to DC, and CB parallel to DA, and the intersection of these lines will determine B, if the field is a parallelogram. If the field is not a parallelogram, retrace one of the lines terminated by known corners, and compare the bearing with the bearing in the original notes, which will give the variation of the needle. Then run the lines AB and CB from the notes, allowing for the variation, and the intersection will determine B. In like manner two or more lost corners may be found. If the bearings and distances are given in the original notes, and but one corner can be found, retrace some established line in the neighborhood to find the variation, and, beginning at the known corner, run the lines from the notes, allowing for the variation. |