North Bearing must be called N. 0° West, an East Bearing N. 90° E., a South Bearing S. 0° E., and a West Bearing S. 90° W.; counting around from N' to N, in the figure, and so onward, "with the Sun."

The reasons for these corrections are apparent from the Figure, in which the dotted lines and the accented letters represent the direction of the needle, and the full lines and the unaccented letters represent the true North w and South and East and West lines.

When the sum of the Variation and the Bearing is directed to be taken, and comes to more than 90°, the sup

W

Fig. 206.

plement of the sum is to be taken, and the first letter changed. When the difference is directed to be taken, and the Variation is greater than the Bearing, the last letter must be changed. A diagram of the case will remove all doubts. Examples of all these cases are given below for a Variation of 8° West.

must be called N. 0° E., a West Bearing N. 90° W., a South Bearing S. 0° W., and an East Bearing S. 90° E., counting from N' to N, and so onward, "against the sun." The reasons for these rules are seen in the Figure. Examples are given below, for a Variation of 5° E.

The compass

(312) To survey a line with true Bearings. may be set, or adjusted, by means of the Vernier, (noticed in Arts. (229) and (237), and shown in Fig. 148, page 126) according to the Variation in any place, so that the Bearings of any lines then taken with it will be their true Bearings. To effect this, turn aside the compass plate, by means of the Tangent Screw which moves the Vernier, a number of degrees equal to the Variation, moving he S. end of the Compass-box to the right, (the North end being supposed to go ahead) if the Variation be Westerly, and vice versa; for that moves the North end of the Compass-box in the contrary lirection, and thus makes a line which before was N. by the neelle, now read, as it should truly, North, so many degrees, West if the Variation was West; and similarly in the reverse case.

CHAPTER VIII.

CHANGES IN THE VARIATION.

(313) The Changes in the Variation are of more practical importance than its absolute amount. They are of four kinds : Irregular, Diurnal, Annual and Secular.

(314) Irregular changes. The needle is subject to sudden and violent changes, which have no known law. They are sometimes coincident with a thunder storm, or an Aurora Borealis, (during which, changes of nearly 1° in one minute, 210 in eight minutes, and 10° in one night, have been observed), but often have no apparent cause, except an otherwise invisible "Magnetic Storm."

(315) The Diurnal change. On continuing observations of the direction of the needle throughout an entire day, it will be found, in the Northern Hemisphere, that the North end of the needle moves Westward from about 8 A. M. till about 2 P. M. over an arc of from 10' to 15', and then gradually returns to its former position.* In the Southern Hemisphere, the direction of this motion is reversed. The period of this change being a day, it is called the Diurnal Variation. Its effect on the permanent Variation is necessarily to cause it, in places where it is West, to attain its maximum at about 2 P. M., and its minimum at about 8 A. M.; and the reverse where the Variation is East.

This Diurnal change adds a new element to the inaccuracies of the compass; since the Bearings of any line taken on the same day, at a few hours interval, might vary a quarter of a degree, which would cause a deviation of the end of the line, amounting to nearly half a link at the end of a chain, and to 35 links, or 23 feet, at the end of a mile. The hour of the day at which any important Bearing is taken should therefore be noted.

* A similar but smaller movement takes place during the night.

(316) The Annual change. If the observations be continued throughout an entire year, it will be found that the Diurnal changes vary with the seasons, being about twice as great in Summer as in Winter. The period of this change being a year, it is called the Annual Variation.

(317) The Secular change. When accurate observations on the Variation of the needle in the same place are continued for several years, it is found that there is a continual and tolerably regular increase or decrease of the Variation, continuing to proceed in the same direction for so long a period, that it may be called the Secular change of Variation.*

The most ancient observations are those taken in Paris. In the year 1541 the needle pointed 7° East of North; in 1580 the Variation had increased to 1110 East, being its maximum; the needle then began to move Westward, and in 1666, it had returned to the Meridian; the Variation then became West, and continued to increase till in 1814 it attained its maximum, being 22° 34′ West of North. It is now decreasing, and in 1853 was 20° 17' W. In London, the Variation in 1576 was 11° 15' E.; in 1662, 0°; in 1700, 9° 40′ W.; in 1778, 22° 11′ W.; in 1815, 24° 27′ W.; and in 1843, 23° 8' W.

In this country the north end of the needle was moving Eastward at the earliest recorded observations, and continued to do so till about the year 1810 (variously recorded as from 1793 to 1819), when it began to move Westward which it has ever since continued to do. Thus, in Boston, from 1708 to 1807 the Varia tion changed from 9° W. to 6° 5' W., and from 1807 to 1840, it changed from 6° 5′ W. to 9° 18′ W.

Valuable Tables of the Secular changes of the Variation in vari ous parts of the United States have been published by Prof. Loomis in Silliman's "American Journal of Science," Vol. 34, July, 1838, p. 301; Vol. 39, Oct. 1840, p. 42; and Vol. 43, Oct. 1842, p. 107. An abstract of the most reliable of them is here given. Troy and Schenectady are from other sources.

*If the term "Declination of the Needle" could be restored to its proper use, this "Change of Variation” would be properly called the " Variation of the De clination."

From these and other observations it appears that at present the lines of equal variation are moving Westward, producing an annual change of variation (increasing the Westerly and lessening the Easterly) which is different in different parts of the country, and is about five or six minutes in the North-Eastern States, three or four minutes in the Middle States, and two minutes in the Southern States.

(318) Determination of the change, by Interpolation. To determine the change at any place and for any interval not found in the recorded observations, an approximation, sufficient for most. purposes of the surveyor, may be obtained by interpolation (by a simple proportion) between the places given in the Tables, assuming the movements to have been uniform between the given dates; and also assuming the change at any place not found in the Tables, to have been intermediate between those of the lines of equal variation, which pass through the places of recorded observations on each side of it, and to have been in the ratio of its respective dis