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strument fast, and turn the index to 90 and 270 and then it will be at right angles with the line. So that the small sights, at those of the circle, can be of no additional use to the instrument, and therefore should be laid aside as useless.
This instrument may be used in windy and rainy weather, as well as in mountainous and hilly grounds; for it does not require an horizontal position to find the bearing, or angle, as the needle doth; and therefore is preferred to any instrument that is governed by the needle.
OF THE SEMICIRCLE.
circle, divided from its diameter into 180 degrees, and from thence again, that is, from 0, to 360 degrees : it is generally made of brass, and is from 8 to 18 inches in diameter.
On the centre there is a moveable index with sights, on which is placed a circumferentor-box, as in the theodolite.
This instrument may be used as the theodolite in all respects; but with this difference, when you are to reckon the degree to that end of the index which is off of the semicircle, you may find it at the other end, reckoning the degree from 180 forwards.
PLANE TABLE is an. oblong of oak, or
other wood, about 15 inches long, and 12 broad, they are generally composed of 3 boards, which are easily taken asunder, or put together, for the convenience of carriage.
There is a box frame, with 6 joints in it, to take off and put on as occasion serves; it keeps the table together, and is likewise of use to keep down a sheet of
which is put thereon. The outside of the frame is divided into inches and tenths, which serve for ruling parallels or squares on the paper, or for shifting it, when occasion serves.
The inside of the frame is divided into 360 degrees, which, though unequal on it, yet are the degrees of a circle produced from its centre, or centre of the table, where there is a small hole.
The degrees are subdivided as small as their distance will admit; at every tenth degree are two numbers, one the number of degrees, the other its complement to 360.
There is another centre hole about 1 of the ta. ble's breadth from one edge, and is in the middle between the two ends. To this centre hole on the other side of the frame, there are the divisions of a semi-circle, or 180 degrees; and these again are subdivided into halves, or quarters, as the size of the instrument will admit.
That side of the frame, on which the 360 degrees are, supplies the place of a theodolite, the other, that of a semicircle.
There is a circumferentor-box of wood with a paper chart at the bottom applied to one side of the table by a dove-tail joint, fastened by a screw. This box (besides its rendering the plane table capable of answering the end of a circumferentor) is very useful for placing the instrument in the same position every remove.
There is a brass ruler or index of about two inches broad, with a sharp or fiducial edge, at each end of which is a sight ; on the ruler are scales of equal parts with and without diagonals, and a scale of chords; the whole is fixed on a ball and socket, and set on a three-legged-staff.
To take the angles of a field by the table. Having placed the instrument at the first station turn it about till the north end of the needle be over the meridian, or flower-de-luce of the box, and there screw it fast. Assign any convenient point, to which apply the edge of the index, so as through the sights you may see the object in the last station, and by the edge of the index from the point draw a line. Again, turn about the index with its edge to the same point, and through the sights observe the object in the second station, and from the point, by the edge of the index, draw another line; so is the angle laid down; on that last line set off the distance to the second station, in chains and links; apply your instrument to the second station, taking the angle as before ; and after the like manner proceed till the whole is finished.
This method may be used in good weather, if the needle be well touched and play freely ; but if it be in windy weather, or the needle out of order, it is better, after having taken the first angle as before, and having removed your instrument to the second station, and placed the needle over the meridian line as before, to lay the index on the last drawn line and look backward thro' the sights; if you then see the object in the first station, the table is fixed right, and the needle is true; if not, turn the table about, the index lying on the last line, till thro' the sights you see the object in the first station; and then screw it fast, and keeping the edge of the index to the second station, direct your sights to the next; draw a line by the edge of the index, and lay off the next line ; ceed thro' the whole without using the needle, as you do with the theodolite.
If the sheet of paper on the table be not large enough to contain the map of the ground you survey, you must put on a clean sheet, when the other is full ; and this is called shifting of paper, and is thus performed.
Plate VI. fig. 8.
Let ABCD represent the sheet of paper on the plane table, upon which the plot E, F, G, H, I, K, L, M, is to be drawn ; let the first station be E, proceed as before from thence to F, and to G; then proceeding to H, you find there is not room on your paper for the line GH; however, draw as much of the line GH, as the paper can hold, or draw it to the paper's edge. Move your instrument back to the first station E, and proceed the contrary way to M, and to L"; but in going
from thence to K, you again find your sheet will not hold it; however, draw as much of the line LK on the sheet as it can hold.
Take that sheet off the table, first observing the distance oo of the lines GH and LK, by the edge of the table; take off that sheet, and mark it with No. 1, to signify it to be the first taken off. Having then put on another sheet, lay that distance 00 on the contrary end of the table, and so proceed as before with the residue of the survey, from o to H, to K, and thence to o; so is your survey complete.
In the like manner you may proceed to take off, and put on, as many sheets as are convenient; and these may afterwards be joined together with mouth glue, or fine white wafer, very thin.
If the index be fixed to the first centre, using the 360 side, it will then serve as a theodolite, and when to the second centre, using the 180 side, it will serve as a semicircle; by either of which you may survey in rainy weather, when you cannot have paper on the table.