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OF THE VARIATION OF THE COMPASS AND ATTRACTION OF THE
The Variation of the Compass is the number of Degrees that the Magnetic Needle points from the true North, either East or West. This differs in different places, and in the same place at different times. It is, at present, in Connecticut, a few degrees to the Westward. That is, the Needle points to the Westward of North, and is gradually approaching the true North.
The following method of ascertaining the Variation, by the North Star, has been adopted by many Surveyors, as the most eligible to be practised on Land. It was communicated to the Compiler by Moses Warren, Jun. Esq. of Lyme, an experienced Surveyor, with permission to publish it.
The Star commonly called the North Star, is not directly North but revolves round the Pole in a small circle, once in 24 hours. It cannot therefore be due North but twice in that period; and that is within a very few minutes of the time when a Star, called Alioth, in the Constellation of Ursa Major, or the Great Bear, is directly over or under it. There is also another Star nearly in an opposite direction from the Pole, called Gamma, in the Constellation of Cassiopeia. When these three Stars are vertical the North Star is very near the Meridian; and when they are horizontal, it is at its greatest Elongation, that is, at its greatest distance East or West of the Pole, and on the same side as the Star in Cassiopeia, The Variation may
be calculated when the Star is on the Meridian, or when at its greatest Elongation; more accurately, however, cal for some time gives the observer a better opportunity to complete his observation.*
To find the Elongation of this Star in any Latitude, its Deckination must be known; that is, its distance North of the Equator. This being found, institute the following Proportion:
As Co-Sine of the Latitude; Is to Radius; So is Co-Sine of the Declination; to Sine of the Elongation.
The Declination of the North Star, January 1, 1810, was 880 17' 28", and increasing at the rate of about 19 seconds and one half annually.
The following Table Shows the Elongation, in several different Latitudes, for 5 years successively. It is calculated for the first of January in each year; and in using it, if the time, when the Elongation is required, be past the middle of the year, take it for the beginning of the next year.
A Table showing the Elongation of the North Star.
20 5' 34' 20 5/7 20 4' 39/20 4' 13'20 3'4711
2 13 10 2 12 39 2 12 11 2 11 43 2 11 16
2 15 22 2 14 51 2 14 22 2 13 54 2 13 26
2 17 33 2 17 4 2 16 34 2 16
5 2 15 37
** The following Figure exhibits a view of the relative situation of these
The Elongation for the Latitude of the observation being calculated, or taken from the above Table, proceed to find its r'ange, according to the following directions ;
Take a pole 18 or 20 feet in length; to the end of it fasten a small line; raise it to an elevation of 45° or 500; and support it, by two cratches of suitable height to keep it firm in its place. At the end of the line, near the ground, fasten a weight of half a pound or more, which should swim in water to prevent the air from moving the line.' Southward of the line, fix a Compass sight, or other piece of metal or wood, with a narrow, perpendicular aperture at a convenient height from the ground, say about 2 or 2 1-2 feet; and let it be so fixed that it can be moved a small distance East or West at pleasure. Let an assistant hold a light either NE, or NW.of the line, nearly as high as the range from the sight to the North Star, in such a position that the line may be plainly seen; then, (the three Stars above mentioned being parallel or nearly so with the Horizon) move the sight-vane East or West, until through the aperture, the line is seen to cut the Star; and continue to observe, at short intervals, till the Star is seen at its greatest Elongation. Let a lighted candle be placed in an exact range with the sight-vane and line at the distance of 20 Rods or more, which should stand' perpendicularly, be made fast, extinguished, and left till morning. Then the sight-vane, the line, and the candle, will be the range of Elongation, which observe accurately with a Compass ; and is the Elongation be East and the Variation West, the former must be subtracted from the latter; and if they are both West they must be added, and their difference, or sum will be the true Variation,
OF THE ATTRACTION OF THE NEEDLE.
It is well known that any iron substance has an influence upon the magnetic Needle, attracting it one way or the other from the point where it would settle, were there no such attraction. A surveyor should therefore be careful to see that no iron is near the
compass when 'taking a bearing. But as the Earth in certain spots contains, near its surface, iron or other minerals · which attract the Needle, it will frequently happen that it will point wrong. To ascertain whether this is the case,
the surveyor, at each station, should take a back view of the one last left; and if he finds that the compass does not reverse truly, he
may be sure, provided the compass be accurately graduated and placed horizontally, that he either made a mistake at the last station, or that in one of the other of the stations, the Needle was attracted from the true point. When he finds a place where he suspects there is an attraction, he should go a few rods backward or forward, and see whether the Needle points differently. In this way he may prevent mistakes in his field notes, by putting down a wrong course. To take back sights is particularly necessary in running long lines, and laying out new lands, where the Needle is the only thing to guide the surveyor.
By practice and experience a knowledge will be acquired on this subject, and with regard to many other things in surveying, which cannot be taught by books; and after all the directions which can be written, the practitioner will frequently find occasion for the exercise of his own judgment.
1 Rule to find the difference between the present variation of the
Compass, and that at a time when a Tract was formerly surveyed, in order to trace or run out the original lines.
Go to any part of the premises where any two adjacent corners are known ; and if one can be seen from the other, take their bearing ; which, compared with that of the same line in the former survey, shows the difference, But if one corner cannot be seen from the other, run the line according to the given bearing, and observe the nearest distance between the line so run and the corner; then work by the following proportion:
As the length of the whole line,
Suppose it be required to run a line, which, some years ago, bore N. 450 E., distance 20 chains, and in running this line by the given bearing, the corner is found 20 links to the left hand; what is the present bearing of this line?
* 57.3 degrees is the Radius of a circle (nearly) in such parts as the cir
2000)68760(34 Minutes. Answer--34 Minutes to the left hand is the allowance required, and the line in question bears N. 440 26' E.
The compiler of this work acknowledges himself under obligations to George Gillett, Esq. Surveyor General of the state of Connecticut, for the following illustrations, remarks, and miscellaneous questions, considering them calculated to be useful to the learner, and the practical surveyor. They came to hand too late to be inserted in their proper places, in the body of the work, and are here put together in the Appendix.
By a statute of this state, applicants for the appointment of County Surveyor are required to be well skilled in point of science in the theory of the most approved methods of surveying lands. It is also as necessary that they should be as well skilled in practical surveying. A practical knowledge must be acquired by experience, and no one can have a thorough knowledge of correct practice without being made acquainted with the imperfections and irregularities of the Magnetic Needle.
It is supposed, by most people, that this instrument, in alt places, points directly to the Poles of the earth, and that it remains as permanent as the Poles themselves-an infallible guide.* This is a mistaken idea. A few remarks on this sub
* There is one line around the globe on which there is no variation. The general course of this line, on this side of the globe, is from northwest to southeast, but is crooked and irregular in its course. According to Dr. Holly's cbart, made in 1700, the line of no variation crossed the meridian of London in 550 South latitude-crossed the equatur in 170 W. longi