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To find the area of LABI.
As radius 90 10.00000 Is to LAY 29.47 1.46938 IB 35.43 1,54937 So is sine [I 13.45 9,37600 12.39473
To double area 248.17 2.394.75
To find the area of IBCI.
As radius 90 10,00000 Is to IB 35.43 1.5%937 IC 29.83 1.47465 So is sine [I 9.15 9.20613 12.23015
To double area 169.9 2,23015
As radius 90 10.00000 Is to IC ) 29.83 1.47+65 - ID 30.75 1.487.8% So is sine [I 19.80 9.52350 12.48599 10.00000
To double area 306.2 2.48599
To find the area of IDEI.
As radius 90 * . 10.00000 Is to ID 30.75 1.487.84 IE s 22.23 1.34694 So is sine [I 26.30 9.64953 12.48431
To double area 305 2.48431 *
The small difference between this method, and the former, is owing to some losses sustained in not finding the exact number answering to a logarithm, and to the advantage of two decimal figures in the difference of latitude and departure, in working the traverse table; therefore the first method is preferable.
IT sometimes happens, that the needle is attracted out of its customary position, by an unknown cause; but most probably owing to some matter near the surface of the earth : as it is frequently observable where there is the appearance of iron-stone; and this attraction will frequently vary in quantity, and in denomination, at every station. Therefore, to take the bearings truly, where the needle is attracted, observe the following directions; as for example,
Suppose a field, bounded as by the Map in the following page, to be so situated; a true survey of which is required.
* Mote. Particular care must be taken to cqunt the degrees at every station from the same end of the needle; as in this example, K count from the
Seek a place where two stations can be taken, without attraction; this may be tried and proved by back-sights, and the nearer to one corner of the field the better, whether within or without the inclosure, as for instance; suppose I set my instrument at G, and take the bearing to H, and then by a back-sight from H to G, the bearing is the same ; therefore no attraction at either of these stations. Then from H, I take the bearing to A, one corner of the field, and it is N. 25° W. then by a back-sight H bears
212 OF ATTRACTION.
from A, S 22' 4 E. therefore at A, the north point of the needle is attracted 23 degrees westerly; then turning the sights to B, it bears N. 40° 4 W. to this, add the attraction 23 degrees, and it gives the true bearing N. 43° W. distance measured is 7 chains, 20 links. Then, by a back-sight, A bears from B, S.46° E. therefore the north point is attracted 3 degrees easterly at this place; then