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THE VERNIER COMPASS. This instrument, represented in the engraving opposite, is in most respects like that already described, differing from it mainly in having its compass circle, to which is attached a vernier, movable about a common centre a short distance in either direction, thus enabling the surveyor to set the zeros of the circle at any required angle with the line of sights, the number of degrees contained in this angle or the "varia- tion of the needle" being read off by the vernier. The movement of the circle is effected either by a slow moving or "tangent screw," as shown in the engraving, or by a concealed rack and pinion, the head of which projects from the under side of the main compass plate. When the variation is set off as described, the circle is securely fastened in its position by a clamping nut underneath the main plate.
BALL SPINDLE — The compass is usually fitted to a spindle made slightly conical and having on its lower end a ball turned perfectly spherical, and confined in a socket by a pressure so light that the ball can be moved in any direction in the operation of leveling the compass. The ball is placed either in the brass head of the Jacob Staff already shown with the previous instrument, or still better, in the compass tripod seen in the engraving of the Vernier Transit beyond. The superiority of the vernier over the plain compass consists in its adaptation to the retracing the lines of an old survey, and to the surveys of the U. S public lands, where the lines are based on a true meridian.
VARIATION OF THE NEEDLE. It is well known that the magnetic needle, in almost all parts of the United States, points more or less to the east or west of a true meridian, or north and south line. This deviation, which is called the VARIATION or DECLINATION of the needle, is not constant, but increases or decreases to a very sensible amount in a series of years. Thus at Troy. N. Y., a line bearing in 1838, N. 3l deg. E., would, in 1874, with the same needle, have a bearing of about N. 320 E., the needle having thus in that interval travelled a full degree to the west. For this reason, therefore, in running over the lines of a farm from field notes of some years standing, the surveyor would be obliged to make an allowance, both perplexing and uncertain, in the bearing of every line. To avoid this difficulty the vernier was devised, the arrange ment of which we shall now describe. The Vernier is divided on its edge to thirty equal parts, and figured in two series on each side of the centre line. In the same plane with the vernier is an arc or limb, fixed to the main plate of the compass, and graduated to half degrees. The surfaces of both vernier and limb are silvered. On the vernier are thirty equal divisions, which exactly. correspond in length with thirty-one of the half degrees of the limb. Each division of the vernier is, therefore, one-thirtieth or, in other words, one minute longer than a single division of the limb.
To READ THE VERNIER.—In "reading" the vernier, if it is moved to the right, count the minutes from its zero point to the left, and vice versa. Proceed thus until a division on the vernier is found exactly in line with another on the limb, and the lower row of figures on the. vernier will give the number of minutes passed over. When the vernier is moved more than fifteen minutes to either side the number of the additional minutes up to thirty or one-half degree of the limb is given by the upper row of figures on the opposite side of the vernier. To read beyond thirty, add the minutes given by the vernier to that number, and the sum will be the correct reading. In all cases when the zero point of the vernier passes a whole degree of the limb, this must be added to the minutes, in order to define the distance over which the vernier has been moved.
To TURN OFF THE VARIATION.—It will now be seen that the surveyor having the vernier compass, can by moving the vernier to either side, and with it of course the compass circle attached, set the compass to any variation. He therefore places his instrument on some well defined line of the old survey, and turns the tangent screw until the needle of his compass indicates the same bearing as that given in the old field notes of the original survey. Then screwing up the clamping nut underneath the vernier, he can run all the other lines from the old field notes without further alteration. The reading of the vernier on the limb in such a case would give the change of variation at the two different periods. The variation of the needle at any place being known, a true meridian, or north and south line, may be run by moving the vernier to either side, as the variation is east or west, until the arc passed over on the limb is equal to the angle of variation; and then turning the compass until the needle is made to cut the zeros on the divided circle, when the line of the sights would give the direction of the true meridian of the place. Such a change in the position of the vernier is necessary in surveying the U. S. public lands, which are always run from the true meridian.
THE LINE OF NO VARIATION, as it is called, or that upon which the needle will indicate a true north and south direction, is situated in the United States, nearly in an imaginary line drawn from the middle of lake Erie to Cape Hatteras, on the coast of North Carolina. A compass needle, therefore, placed east of this line would have a variation to the west, and when placed west of the line, the variation would be to the east, and in both cases the variation would increase as the needle was carried farther from the line of no variation. Thus in Minnesota the variation is from 15 deg. to 16 deg. to the east, while in Maine it is from 17 deg to l8 deg. to the west. At Troy, in the present year, 1874, the variation is about 9 deg. to the west, and is increasing in the same direction from two to three minutes annually.
To READ TO MINUTES — A less important use of the vernier is to give a reading of the needle to single minutes, which is obtained as follows: First be sure, as in all observations, that the zero of the vernier exactly corresponds with that of the limb; then noting the number of whole degrees given by the needle, move back the compass circle with the tangent screw until the nearest whole degree mark is made to coincide with the point of the needle, read the vernier as before described, and this reading added to the whole degrees will give the bearing to minutes.
To use the Vernier Compass. Proceed in the same manner as directed in regard to the Plain Compass, when making new surveys, always taking care that the vernier is set at zero and securely clamped by screwing up the nut beneath the plate. In surveying old farms, allowance and correction must be made for the variation, as just described.
Sizes of the Vernier Compass. We make three sizes of this compass, having needles of four, five and six inches long respectively, the main plates of the two largest being over fifteen inches long; and of the smallest size, thirteen inches, the sights of the last are also about an inch shorter. In the four and five inch Vernier Compasses, the variation arc is within the compass circle like that of the railroad compass hereafter described, and the variation is set off to minutes by a pinion head underneath the plate; the circle is also clamped at any variation by a screw placed opposite the pinion.
Weight of the Vernier Compasses. The average weights of the different sizes, including the brass head of the Jacob Staff, beginning with the smallest, are respectively 5 1/2, 7 1/2 and 9 1/2 pounds.
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