9.51

6.91

.01

.01

9.50

6.90

190.0950

16.07

.01

.01

16.06

.01

210.7072

13.13 E.

6.98

.11 Er. S.

.10 Er. E.

2)3050.7960

Half

do.

(sq. ch.)

762.6 90

To fin

ad the latitude and departure of CI, and area of the

part IABCI.

Hence IN bears, N. 60° 52′ E. dist. 28.3

10.31261

1.13988

1.45249

5 ch.

1 as follow

2nd. N. 4!

¡° W. 24.36

18 E. 16 ch

2. Given the boundaries of a tract of lan viz. 1st. S. 35° W. 11.20 ch. ch. 3rd. N. 15° E. 10.80 ch 4th. S. 7: 5th. N. 87°, E. 21.50 ch. 6th. S. 60o E. 14 South, 10.91 ch. 8th. N. 85 W. 29.28 ch of beginning; to divide the tract into two e a line running from the first station and falli the opposite sides; the bearing and distanc

.80 ch. 7th

to the plac qual parts by ng on one o of the divi

A

in ge to the

some

ward.

TI

that I

the e

ΤΙ

needl north

As

in the sensi a lin

beari

time

cause

meridian pointed out by the magnetic needle is not neral a true one; for the needle does not point truly e north point of the horizon, but varies from it, in places to the eastward, and in others to the west

e angle contained between the true meridian and pointed out by the needle, is called the variation of ompass.

e variation is named east or west, according as the e points to the eastward or westward of the true

the variation is different in different places, so also same place it does not remain the same, but differs bly in the course of a few years. Hence in running that was run a number of years previously, the ng will be found different from what it was at that ; this, together with the difference in compasses, es many difficulties, and frequently inaccuracies, in

1

1

VARIATION OF THE COMPASS:

193

The easiest way to guard against those difficulties and inaccuracies would be to make and return the surveys according to the true and not the magnetic bearings. In order to do this it will be necessary to know the variation of the compass for the place in which the survey is made; and this may readily be found by first tracing a meridian line in the following manner.

To draw a true meridian line by means of the greatest elongation of the pole-star.

The pole-star is situated about 1° 41' from the true pole, and therefore apparently revolves round it, in a small circle, once in about 23 h. 56 m. When at its greatest distance east or west from the true pole, it is said to be at its greatest east or west elongation. It is therefore evident that in the course of one apparent revolution it must be twice at its greatest elongation, once to the east and once to the west.

The following tables exhibit the times, nearly, of the greatest eastern elongations of the pole-star for six months of the year, and of the greatest western elongations for the other six months. The other greatest elongations take place in the day time, and are therefore invisible. Some of those inserted in the tables are also invisible; because they occur, either before daylight is gone in the evening, or after it has returned in the morning. the most of those in the 3rd, 4th, 9th and 10th months are in this situation.

The time in the tables is reckoned from noon, and therefore when it is less than 12 hours, the greatest elongation takes place in the evening of the same day; but when it exceeds 12 hours, if 12 hours be subtracted from it, the remainder will be the time of greatest elongation in the morning of the following day.