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Suppose AB to be the breadth of a river, or
any other inaccessible distance, which may be required.
Let a staff or any other object be set at B, draw yourself backward to *} convenient distance C, so that B may cover 4: from B, lay off any other distance by the river's side to E, and complete the parallelogram EBCD: stand at D, and cause a mark to be set at F, in the direction of A ; measure the distance in links from E to F, and FB will be also given. Wherefore EF: ED : ; FB : AB. Since it is plain (from part 1. theo. 3. sect. 4. and theo. 2. sect. 4.) the triangles EFD and BFA are mutually equiangular.
If part of the chain be drawn from B to C, and the other part from B to E, and if the ends at E and C be kept fast, it will be easy to turn the chain over to D, so as to complete a parallelogram ; by reckoning off the same number of links you had in BC, from E to D, and pulling each part straight.
THIS instrument is composed of a brass circular box, about five or six inches in diameter; within which is a brass ring, divided on the top into 360 degrees, and numbered 10, 20, 30, &c. to 360: in the centre of the box is fixed a steel pin finely pointed, called a centre-pin, on which is placed a needle touched by a loadstone, which always retains the same situation; that is, it always points to the North and South points of the horizon nearly, when the instrument is horizontal, and the needle at rest.
The box is covered with a glass lid, in a brass rim, to prevent the needle being disturbed by wind or rain, at the time of surveying: there is also a brass lid or cover, which is laid over the former to preserve the glass in carrying the instrument. -
This box is fixed by screws, to a brass index, or ruler, of about 14 or 15 inches in length, to the ends whereof are fixed brass sights, which are screwed to the index, and stand perpendicular thereto : in each sight is a large and a small aperture, or slit, one over the other; but these are changed, that is, if the large aperture be uppermost in the one sight, it will be lowest in the other, and so of the small ones: therefore the small aperture in one is opposite to the large one in the other; in the middle of which last, there is placed a horse hair, or fine silk thread. |
The instrument is then fixed on a ball and socket; by the help of which and a screw, you can readily fix it horizontally in any given direction; the socket being fixed on the head of a three-legged staff, whose legs, when extended, support the instrument whilst it is used.
Let your instrument be fixed at any angle as .A., your first station; and let a person stand at the next angle B, or cause a staff, with a white sheet, to be set there perpendicularly for an object to take your view to: then having placed your instrument horizontally (which is easily done by turning the box so that the ends of the needle may be equidistant from its bottom, and it traverses or plays freely) turn the flower-deluce, or north part of the box, to your eye, and looking through the small aperture, turn the index about, till you cut the person or object in the next angle B, with the horse hair, or thread of the opposite sight; the degrees then cut by the south end of the needle, will give the number to be placed in the second column of your field-book in a line with station No. 1, and expresses the number of degrees the stationary line is from the north, counting quite round with the sun.
Most needles are pointed at the south end, and have a small ring at the north: such needles are
better than those which are pointed at each end,
because the surveyor cannot mistake by counting to a wrong end; which error may be frequently committed, in using a two-pointed needle.
Two-pointed needles have sometimes a ring, but more usually a cross towards the north end: and the south end is generally bearded towards its extremity, and sometimes not, but its arm is a naked right line from the cap at the centre.
Having taken the degrees or bearing of the first stationary line AB, let the line be measured, and the length thereof in chains and links be inserted in the third column of your field-book, under the title of distances, opposite to station No. 1.
It is customary, and even necessary, to cause a
sod to be dug up at each station, or place where you fix the instrument: to the end, that if any error should arise in the field-book, it may be the more readily adjusted and corrected, by trying over the former bearings and stationary distances.
Having done with your first station, set the instrument over the hole or spot where your object stood, as at B, for your second station, and send him forward to the next angle of the field, as at C; and having placed the instrument in an horizontal direction, with the sights directed to the object at C, and the north of the box next your eye, count your degrees to the south end of the needle, which register in your field-book, in the second column opposite to station No. 2; then measure the stationary distance BC, which insert in the third column, and thus proceed from angle to angle, sending your object before you, till you return to the place where you began, and you will have the field-book complete; observing always to signify the parties’ names who hold the contiguous lands, and the names of the townlands, rivers, roads, swamps, lakes, &c. that bound the land you survey, as before; and this is the manner of taking field-notes by what is called fore-sights.
But the generality of mearsmen frequently set themselves in disadvantageous places, so as often to occasion two or more stations to be made, where one may do, which creates much trouble and loss of time; we will therefore show how this may be remedied, by taking back-sights, thus: let your object stand at the point where you begin your survey, as at A ; leaving him there, proceed to your next angle B, where fix your instrument so, that you may have the longest view possible towards C. Having set the instrument in an horizontal position, turn the south part of the box next your eye, and having cut your object at A, reckon the degrees to the south point of the needle, which will be the same as if they were taken from the object to the instrument, the direction of the index being the same. Let the degree be inserted in the field-book, and the stationary distance be measured and annexed thereto, in its proper column; and thus proceed from station to station, leaving your object in the last point you left, till you return to the first station A.
By this method your stations are laid out to the best advantage, and two men may do the business of three, for one of those who chain, may be your object; but in fore-sights, you must have an object before you, besides two chainmen,