dian.

Thus, township 1 North, range 4 West, has its exact place designated, and may be immediately located.

113. The principal meridians, and the principal base-lines are established by astronomical observation, and the lines of, subdivision run with the compass.

For convenience in making surveys, and for the purpose of designating particular localities, a state or large tract, is often divided into parts called "Districts." There are three

such districts in the map before us, the Lawrence County District, the Arkansas District, and the Mississippi District, the boundaries of which are designated by the full dark lines.

114. Each township is divided into equal squares, by meridians one mile apart, and by east and west lines at the same distance from each other. Hence, each township is divided into 36 square miles, each one of which is called a section. The sections of a township are numbered from 1 to 36, beginning at the northeast angle, and each contains 640 acres.

The diagram exhibits the 36 sections of a township.

To describe a section accurately, we say, section number 5, in township number 4 north, in range 3d west of a

known meridian; the one, for example, drawn through the mouth of White river. The description fixes precisely the place of the section. Go to the 3d range of townships, west of the known meridian, find township number 4 north, in this range, and lastly, section number 5 of that township. The corners of the sections should be marked by permanent corner-posts, or by lines blazed on trees.

115. The sections are divided into half sections, quarter sections, and even into eighths of sections. The following table shows the contents of a township, and its subdivisions: 1 township = 36 sections = 23040 acres.

1 section = 640 acres.

section = 320 acres.

section 160 acres.

section = 80 acres.

VARIATION OF THE NEEDLE.

116. The angle which the magnetic meridian makes with the true meridian, at any place on the surface of the earth, is called the variation of the needle at that place.

The variation is east, when the northern end of the needle, after settling to a state of rest, lies on the east of the true meridian; and west, when it lies on the west side of that meridian.

117. The variation is different at different places, and even at the same place it does not remain constant for any length of time. The variation is ascertained by comparing the magnetic with the true meridian.

118. If we suppose a line to be traced through those points, where the needle points directly north and south, such

By referring to a map of the United States, such line is easily traced; for, in the year 1870, it passed, very nearly, through Raleigh, in the State of North Carolina, Cleveland, in the State of Ohio, and crossed the Saut of St. Mary's at the lower end of Lake Superior. If a compass, at that time, had been placed anywhere on this line, the needle would have pointed due north and south: hence, this line was then the line of no variation.

At all points on the surface of the earth, the north end of the needle inclines toward the line of no variation: hence, for all points east of this line, the variation is West; and for all points west of it, the variation is East.

119. The table on the last page, and the one which follows, on the next, haye been constructed from the magnetic chart accompanying the Annual Report of the Coast Survey of 1865, and all the magnectic meridians are calculated for the year

1870.

On that chart, we find the meridian of no variation. It passes through Raleigh, in the state of North Carolina (very nearly), and through Cleveland, in the state of Ohio. East of it, is marked the magnetic meridian of one degree: that is, the magnetic meridian, at any point of which, the variation. is 1 degree west. Two points of this meridian are noted in the table: viz., the point whose latitude is 36° north, and longitude 77° 20' west; and also the point whose latitude is 40° 30', and longitude 80°. Marking these two points, on a map of the United States, the magnetic meridian of one degree variation, west, may be drawn. And in a similar manner any magnetic meridian (of which two points are indicated by the table), may be drawn.

1

To aid in the ready selection of any point, indicated in the table, the prominent place, nearest to it, is written in the adjoining column; and when the place lies east of the point (and consequently the point west of the place), the signis annexed; and the sign, when the place lies west of the point, and the sign , when the meridian passes through, or very near the place. Thus, in the meridian of

1o variation, Plymouth, in North Carolina, is east of the first

This table is interpreted like the preceding. At San Diego, on the Pacific, the variation is 13° 25′ East; at San Francisco, it is 16° 30'; and at the mouth of the Columbia river, it is 21° 10' East.

METHODS OF ASCERTAINING THE VARIATION.

120. The best practical method of determining the true meridian of a place, is by observing the north star. If this star were precisely at the point in which the axis of the earth, prolonged, pierces the heavens, then, the intersection of the vertical plane passing through it and the place, with the surface of the earth, would be the true meridian. But, the star being at a distance from the pole, equal to 1° 30' nearly, it performs a revolution about the pole in a circle, the polar distance of which is 1° 30', nearly.

To the eye of an observer, this star is continually in motion, and is due north but twice in 24 hours; and is then said to be on the meridian. Now, when it departs from the meridian, it apparently moves east or west, for 12 hours, and then returns to the meridian again. When at its greatest