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The variation at West Point in September, 1835, was 6° 32′ west. The variation of the needle should always be noted on every survey made with the compass, and then if the land be surveyed at a future time the old lines can always be rerun.

It has been found by observation that heat and cold sensibly affect the magnetic needle, and that the same needle will at the same place indicate different lines at different hours of the day.

If the magnetic meridian be observed early in the morning, and again at different hours of the day, it will be found that the needle will continue to recede from the meridian as the day advances, until about the time of the highest temperature, when it will begin to return, and at evening will make the same line as in the morning. This change is called the diurnal variation, and varies, during the summer season, from one-fourth to one-fifth of a degree.

A very near approximation to a true meridian, and consequently to the variation, may be had, by remembering that the pole star very nearly reaches the true meridian when it is in the same vertical plane with the star Alioth in the tail of the Great Bear, which lies nearest the four stars forming the quadrilateral.

The vertical position can be ascertained by means of a plumb line. To see the spider's lines in the field. of the telescope at the same time with the star, a faint light should be placed near the object glass. When the plumb line, the star Alioth, and the north star fall on the vertical spider's line, the horizontal limb is firmly clamped and the telescope brought down to the horizon; a light, seen through a small aperture in a board, and held at some distance by an assistant, is then moved according to signals, until it is covered by the intersection of the spider's lines. A picket driven. into the ground, under the light, serves to mark the meridian line for reference by day, when the angle formed by it and the magnetic meridian may be measured.

Table showing the difference of latitude and departure in running 80 chains at any course

from 1 to 60 minutes.

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SPECIMEN FIELD NOTES.

No. 1.

TITLE PAGE.

FIELD NOTES
FIELD

OF THE SURVEY OF THE

THIRD STANDARD PARALLEL NORTH

THROUGH

Range No. 21 East

OF THE

PRINCIPAL BASE AND MERIDIAN,

IN THE

TERRITORY OF MONTANA,

AS SURVEYED BY

JAMES M. PAGE,

U. S. DEPUTY SURVEYOR,

UNDER HIS CONTRACT No. 97,
DATED JULY 23, 1880.

Survey commenced August 23, 1880.
Survey completed August 24, 1880.

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