« PreviousContinue »
METHODS OF ASCERTAINING THE VARIATION.
14. The best practical method of determining the true meridian of a place, is by observing the north star. If this star were precisely at the point in which the axis of the earth, prolonged, pierces the heavens, then, the intersection of the vertical plane passing through it and the place, with the surface of the earth, would be the true meridian. But, the star being at a distance from the pole, equal to 1° 30' nearly, it performs a revolution about the pole in a circle, the polar distance of which is 1° 30': the time of revo lution is 23 h. and 56 min.
To the eye of an observer, this star is continually in motion, and is due north but twice in 23 h. 56 min.; and is then said to be on the meridian. Now, when it departs from the meridian, it apparently moves east or west, for 5 h. and 59 min., and then returns to the meridian again. When at its greatest distance from the meridian, east or west, it is said to be at its greatest eastern or western elongation. The following tables show the times of its greatest eastern and western elongations:
Days. April. May. June. July. August.
The eastern elongations are put down from the first of April to the first of October; and the western, from the first of October to the first of April; the time is computed from 12 at noon. The western elongations in the first case, and the eastern in the second, occurring in the daytime, cannot be used. Some of those put down are also invisi ble, occurring in the evening, before it is dark, or after daylight in the morning. In such case, if it be necessary to determine the meridian at that particular season of the year, let 5 h. and 59 min. be added to, or subtracted from, the time of greatest eastern or western elongation, and the observ ation be made at night, when the star is on the meridian.
15. The following table exhibits the angle which the meridian plane makes with the vertical plane passing through the pole-star, when at its greatest eastern or western elongation such angle is called the azimuth. The mean angle only is put down, being calculated for the first of July of each year:
Lat. 32° Lat. 34° Lat. 36° Lat. 38° Lat. 40° Lat. 42° Lat. 44°
1851 1° 45' 1° 48′ 1° 50' 1° 53' 1° 5632° 001/ 2° 041' 1852 1° 45′ 1° 47'1° 50′ 1° 53′ 1° 5611° 593/2° 033/ 1853 1° 441° 47′ 1° 4931° 52' 1° 55' 1° 5912° 031 1854 1° 44'1° 46' 1° 4911° 52′ 1° 5511° 59' 2° 023 1855 1° 4331° 4611° 483 1° 5131° 5431° 58'2° 021 1856 1° 4311° 45′ 1° 48′ 1° 511 1° 54′ 1° 58′ 2° 013 1857 1° 43' 1° 45'1° 48′ 1° 5031° 54' 1° 57' 2° 011 1858 1° 42' 1° 4431° 47' 1° 50' 1° 53' 1° 57' 2° 00′ 1859 1° 42′ 1° 44′ 1° 47′ 1° 4931° 53′ 1° 56'2° 001' 1860 1° 4131° 44′ 1° 46' 1° 49 1° 52' 1° 56′ 2° 00'
The use of the above tables, in finding the true meridian, will soon appear.
TO FIND THE TRUE MERIDIAN WITH THE THEODOLITE.
16. Take a board, of about one foot square, paste white paper upon it, and perforate it through the centre; the diameter of the hole being somewhat larger than the diameter of the telescope of the theodolite. Let this board be so fixed to a vertical staff, as to slide up and down freely: and let a small piece of board, about three inches square, be nailed to the lower edge of it, for the purpose of holding a candle.
About twenty-five minutes before the time of the greatest eastern or western elongation of the pole-star, as shown by the tables of elongations, let the theodolite be placed at a convenient point and levelled. Let the board be placed about one foot in front of the theodolite, a lamp or candle placed on the shelf at its lower edge; and let the board be slipped up or down, until the pole-star can beseen through the hole. The light reflected from the paper will show the cross hairs in the telescope of the theodolite.
Then, let the vertical spider's line be brought exactly upon the pole-star, and, if it is an eastern elongation that is to be observed, and the star has not yet reached the most easterly point, it will move from the line towards the east, and the reverse when the elongation is west.
At the time the star attains its greatest elongation, it will appear to coincide with the vertical spider's line for some time, and then leave it, in the direction contrary to its former motion.
As the star moves towards the point of greatest elonga tion, the telescope must be continually directed to it, by means of the tangent-screw of the vernier plate; and when the star has attained its greatest elongation, great care should be taken that the instrument be not afterwards moved.
Now, if it be not convenient to leave the instrument in
lamp upon its upper extremity, be arranged at thirty or forty yards from the theodolite, and in the same vertical plane with the axis of the telescope. This is easily effected, by revolving the vertical limb about its horizontal axis without moving the vernier plate, and aligning the staff to coincide with the vertical hair. Then mark the point directly under the theodolite; the line passing through this point and the staff, makes an angle with the true meridian equal to the azimuth of the pole-star.
From the table of azimuths, take the azimuth corresponding to the year and nearest latitude. If the observed elongation was east, the true meridian lies on the west of the line which has been found, and makes with it an angle equal to the azimuth. If the elongation was west, the true meridian lies on the east of the line: and, in either case, laying off the azimuth angle with the theodolite, gives the true meridian.
TO FIND THE TRUE MERIDIAN WITH THE COMPASS.
17. 1. Drive two posts firmly into the ground, in a line nearly east and west; the uppermost ends, after the posts are driven, being about three feet above the surface, and the posts about four feet apart: then lay a plank, or piece of timber three or four inches in width, and smooth on the upper side, upon the posts, and let it be pinned or nailed, to hold it firmly.
2. Prepare a piece of board four or five inches square, and smooth on the under side. Let one of the compasssights be placed at right angles to the upper surface of the board, and let a nail be driven through the board, so that it can be tacked to the timber resting on the posts.
3. At about twelve feet from the stakes, and in the direction of the pole-star, let a plumb be suspended from the top of an inclined stake or pole. The top of the pole should be of such a height that the pole-star will appear about six inches below it; and the plumb should be swung
This being done, about twenty minutes before the time of elongation, place the board, to which the compass-sight is fastened, on the horizontal plank, and slide it east or west, until the aperture of the compass-sight, the plumbline, and the star, are brought into the same range. Then if the star depart from the plumb-line, move the compasssight, east or west, along the timber, as the case may be, until the star shall attain its greatest elongation, when it will continue behind the plumb-line for several minutes; and will then recede from it in the direction contrary to its motion before it became stationary. Let the compasssight be now fastened to the horizontal plank. During this observation it will be necessary to have the plumb-line lighted this may be done by an assistant holding a candle near it.
Let now a staff, with a candle or lamp upon it, be placed at a distance of thirty or forty yards from the plumb-line, and in the same direction with it and the compass-sight. The line so determined, makes, with the true meridian, an angle equal to the azimuth of the pole-star; and, from this line, the variation of the needle is readily determined, even without tracing the true meridian on the ground.
Place the compass upon this line, turn the sights in the direction of it, and note the angle shown by the needle. Now, if the elongation, at the time of observation, was west, and the north end of the needle is on the west side of the line, the azimuth, plus the angle shown by the needle, is the true variation. But should the north end of the needle be found on the east side of the line, the elongation being west, the difference between the azimuth and the angle would show the variation: and the reverse when the elongation is east.
1. Elongation west, azimuth
North end of the needle on the west, angle 4° 06′