Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

veyor 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 degree as 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 disfances, 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 instru'ment 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 fielk-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 town-lands, rivers, roads, swamps, lakes, &c. that bound the land you survey, as before : and this is he manner of taking field-notes by what are called foresights.

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 ma 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

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.

same.

By this method your stations are laid out to the best advantage, and two men may do the business of three,

in fore-sights, you must have an object before you, besides two chainmen.

It was said before, that a surveyor should have a person with him to cary the hinder end of the chain, on whom he can depend : this person should be expert and ready at taking off-sets, as well as exact in giving a faithful return of the length of every stationary line. One who has such a person, and who uses back-sights, will be able to go over near double the ground he could in the same time, by taking fore-sights. But if the surveyor has no such person on whom he can with safety depend, he must take fore-sights, because of overseeing the chaining; for should be take backsights, he must be obliged, after taking his degree, to go back to the foregoing station, to oversee the chaining, and by this means, to walk three times over every line, which is a slavery not to be borne,

Or a back and a fore-sight may be taken at one station, thus ; with the south of the box to your eye, observe from B the object A, and set down the degree in your field-book, cut by the south end of the needle. Again from B observe an object at 0, with the north of the box to your eye, and set down the degree cut by the south point of the needle, so have you the bearings of the lines AB and BC; you may then set up your instrument at D, from whence take a back-sight to C, and a fore-sight to E; thus the bearings may be taken quite round, and the stationary distances being annexed to them, will complete the field-book,

[ocr errors]

But in this last method, care must be taken to see that the sights have not the least cast on either side ; if they have, it will destroy all ; and yet with the same sights, you may take a survey by fore-sights or by backs-sights only, with as great a truth as if the sights were ever so erect, provided the same cast continues without alteration; but upon the whole, back-sights only will be found the readiest method.

If your needle be pointed at each end, in taking fore-sights, you may turn the north part of the box to your eye, and count your degrees to the south part of the needle, as before; or you may turn the south of the box to your eye, and count your degrees to the north end of the needle.

But in back-sights you may turn the north of the box to your eye, and count your degrees to the north point of the needle; or you may turn the south of the box to your eye, and count your degrees to the south end of the needle.

The brass ring in the box is divided on the side into 360 degrees, thus : from the north to the east into 90, from the north to the west into 90, from the south to the east into 90, and from the south to the west into 90 degrees; so the degrees are numbered from the north to the east or west, and from the south to the east or west.

The manner of using this part of the instrument is thus; having directed your sights to the object, whether

of your compass the point of the needle lies between (the north, south, east, and west, being called the four cardinal points, and are graved on the bottom of the box) putting down those points together by their initial letters, and thereto annexing the number of degrees, counting from the north or south, as before, thus; if the point of your needle lies between the north and east, north and west, south and east, or south and west points in the bottom of the box, then put down NE, NW, SE, or SW, annexing thereto the number of degress cut by the needle on the side of the ring, counting from the north or south, as before.

But if the needle point exactly to the north, south, east, or west, you are then to write down N, S, E, or W, without annexing any degrees.

This is the manner of taking field-notes, whereby the content of ground may be universally determined by calculation; and they are said to be taken by the quartered compass, or by the four nineties. To find the number of begrees contained in any given

angle.

Set up your instrument at the angular point, and thence direct the sights along each leg of the angle, and note down their respective bearings, as before ; the difference of these bearings, if less than 180, will be the quantity of degrees contained in the given angle; but if more, take it from 360, and the remainder will be the degrees contained in the given angle.

« ΠροηγούμενηΣυνέχεια »