In the interior of the compass-box, there is a graduated circle divided to degrees, and sometimes to half degrees: the degrees are numbered from the extremities of the diameter NS, both ways to 90°.

The length of the magnetic needle is a little less than the diameter of the graduated circle, so that the needle can move freely around its centre, within the circle, and its positions be noted on the graduated arc.

The compass-box is turned about its centre, without moving the plate AB, by means of the milled screw L: it is fastened to the plate AB, by the screw P.

In using the compass, it is important to ascertain the exact angle which may be included between the magnetic meridian and the direction that may be given to the line drawn through the eye and the sights AF and BG.

To effect this, a small arc HI is described on the bar AB, having its centre at the centre of the compass-box. This arc is divided to degrees, and sometimes to the parts of a degree. A vernier is also used, which is permanently attached to the compass-box.

When the 0 point of this vernier coincides with the 0 point of the graduated arc HI, the line of the compass-box marked NS, lies in the plane of the sights.

Now, supposing the 0 of the vernier to coincide with the 0 of the arc HI, if the end of the needle does not stand at one of the lines of division of the graduated circle, let the whole degrees be read. Then, turn the compass-box by means of the screw L, until the needle points exactly to the line which marked the whole degrees: the space passed over by the 0 of the vernier, shows the parts of a degree that are to be added to give the true reading.

SURVEYING WITH THE COMPASS.

2. The line about which the earth revolves is called its axis, and the two points in which the axis meets the surface of the earth, are called the poles.

plane, and its intersection with the surface is called a me ridian line or a meridian.

4. All the meridians converge towards the poles, but they vary so little from parallelism within the narrow limits of surveys made with the compass, that they may, without error, be regarded as parallel straight lines.

5. If a magnetic needle be suspended freely and allowed to settle to a state of rest, a vertical plane passed through its axis is called the plane of the magnetic meridian; and its intersection with the surface of the earth is called the magnetic meridian, or sometimes a North and South line. A line perpendicular to a North and South line is called an East and West line.

6. A line traced or measured on the ground, is called a course; and the angle which this line makes with the meridian passing through the point of beginning, is called the bearing.

Thus, if we start from the point A, and measure in the direction AB, the line AB is the course, and the angle NAB is the bearing.

W

D

N

B

S

-E

F

When the course, like AB, falls between the north and east points, the bearing is read, north 46° east, and is written N. 46° E.

When the course, like AC, falls between the north and west points, the bearing is read, north 30° west, and is written N. 30° W.

When the course, like AF, falls between the south and east points, the bearing is read, south 70° east, and is written S. 70° E.

When the course, like AD, falls between the south and west points, the bearing is read, south 70° west, and is written S. 70° W.

A course which runs due north, or due south, is desig nated by the letter N or S; and one which runs due east,

7. If, after having passed over a course, the bearing is taken to the back station, this bearing is called the back sight, or reverse bearing.

8. The perpendicular distance between the east and west lines drawn through the extremities of a course, is called the northing or southing, according as the course is run towards the north or south. This distance is also called the difference of latitude, or simply the latitude, because it shows the distance which one of the points is north or south of the other.

Thus, in running the course from A to B, AC is the difference of latitude, north.

9. The perpendicular distance be- W tween the meridians passing through the extremities of a course, is called the departure of that course, and is east or west, according as the course lies on

N

H

C

B

G

A

S

E

the east or west side of the meridian passing through the point of beginning.

east.

Thus, in running the course AB, CB is the departure,

10. It will be found convenient, in explaining the rules for surveying with the compass, to attribute to the latitudes and departures the algebraic signs, + and

We shall, therefore, consider every northing as affected with the sign +, and every southing as affected with the sign We shall also consider every easting as affected with the sign +, and every westing as affected with the sign

11. The meridian distance of a point is its perpendicular distance from an assumed meridian. Thus, if the distance be estimated from the meridian NS, BC will be the meridian distance of the point B.

12. The meridian distance of a line is the meridian distance of its middle point, and is east or west, according as

ridian. Thus, FG drawn through the middle point of AB, the meridian distance of the line AB.

The sign + will always be given to the meridian distance of a point or line, when it lies on the east of the assumed meridian, and the sign -, when it lies on the west.

13. When a piece of ground is to be surveyed, we begin at some prominent corner of the field, and go entirely around the land, measuring the lengths of the bounding lines with the chain, and taking their bearings with the It is not material whether the ground be kept compass. on the right hand or on the left, and all the rules deduced for one of the cases, are equally applicable to the other. To preserve uniformity, however, in the language of the rules, we shall suppose the land to be always kept on the right hand of the surveyor.

Place the compass at A, and take the bearing to B, which is PAB: suppose this angle has been found to be

ter this bearing in the field notes opposite station 1. Then measure the distance from A to B, which we will suppose to be 10 ch., and insert that distance opposite station 1, in the column of distances.

We next take the bearing from В to C, N. 623 E., and then measure the distance BC=9 ch. 25 1., both of which we insert in the notes opposite station 2.

At station C we take the bearing to D, S. 36° E., and then measure the distance CD=7 ch. 60 1., and place them in the notes opposite station 3.

At D we take the bearing to A, S. 451° W., and measure the distance DA=10 ch. 40 1. We shall then have made all the measurements on the field which are necessary to determine the contents of the ground.

15. REMARK I. The reverse bearing or back sight, from B to A, is the angle ABH; and since the meridians NS and HG are parallel, this angle is equal to the bearing NAB. The reverse bearing is, therefore, S. 311° E.

The reverse bearing from C, is S. 623° W.; that is, it is the angle ICB=GBC.

And generally, a reverse bearing, or back sight, is always equal to the forward bearing, and differs from it in both of the letters by which it is designated.

16. REMARK II. In taking the bearings with the compass, there are two sources of error. 1st. The inaccuracy of the observations: 2d. Local attractions, or the derangement which the needle experiences when brought into the vicinity of iron-ore beds, or any ferruginous substances. To guard against these sources of error, the reverse bearing should be taken at every station: if this and the forward bearing are of the same value, the work is proba bly right; but if they differ considerably, they should both be taken again.

17. REMARK III. If the forward and back sights at the end of any course of the survey agree, it may be safely assumed, that no local attraction disturbs the needle at these points; and hence, that the next foresight is also free