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As sine course 33° 20'
9.739975 1.855519 10.000000
: Distance IC 130.5
Note. In this way the course and distance may be found from any
angle of a field to another. Having found the line C I, divide 3470, the number of rods to be contained in the triangle IC N, by one half the line CI, viz. 65.25 ; the quotient will be the length of the perpendicular P N, viz. 53.18.
Now, by the bearings of CI and C D, it appears that they form an angle of 60° 20'; wherefore, in the triangle CPN, are given the side P N 53.18, and the angle at C 60° 20', to find the hypothenuse C N.
As sine PCN 60° 20'
9.938980 1.725748 10.000000
: hyp. C N 61.2
Thus the dividing line must go from I to a point on the line C D, which is 61.2 rods from C. The bearing and dis. tance of this line may be found by the directions given above for finding the bearing and distance of the line CI. Or, they may be found by oblique trigonometry, CASE III.
ANOTHER METHOD OF FINDING THE DISTANCE C N.
Having ascertained the latitude and departure of the line C I, set them down in a traverse table; find the latitude and departure of the line C D, and place them in the table; the difference between the northing of the line IC, and the southing of the line C D, will be the southing of the line DI, viz. 6.6; and the sum of the eastings of those lines, as they are both easterly, will be the westing of the line DI, viz. 123.9. Proceed to calculate the area of the triangle ICD, which will be found to be 6522 rods, nearly. NOTE. As in this triangle two sides and their contained angle are
Having found the area of this triangle, proceed to find CN according to PROB. II. page 75, as follows:
As the area of the triangle; js to C D the base; so is the quantity to be contained in the triangle ICN; to C N, its proportion of the base.
As 6522: 115: : 3470 : 61.19.
A THIRD METHOD OF FINDING THE DISTANCE C N.
To the logarithm of double the area to be contained within the triangle IC N, add radius; from this sum substract the logarithmic sine of the angle at C; and from the remainder substract the logarithm of the side IC; the last remainder will be the logarithm of the side C N.
The double area of the triangle IC N, is 6940; the angle at C is 60° 20'; the side IC is 130. Double area 6940
Note. Radius may be added by placing a unit before the index of
the logarithm for the double area, without the trouble of setting down the ciphers.
BY NATURAL SINES.
Divide the double area by the natural sine of the given angle, and that quotient by the given side; the last quotient will be the side CN.
Nat. sine of the angle at C 600 20' 0.86892
7986.92+130.5=61.2 * The log. of IC, as found by calculation on page 77, is employed, instead of taking from the tables, that of 130.5, which is not the exací
From the above the following general rule may be drawn.
TO FIND THE SIDE OF A TRIANGLE WHEN THE AREA IS GIVEN, AND ALSO ONE OF THE SIDES, AND THE ANGLE CON: TAINED BETWEEN THE GIVEN SIDE AND THE SIDE REQUIRED
TO THE LOGARITHM OF DOUBLE THE AREA ADD RADIUS; FROM THIS SUM SUBSTRACT THE LOGARITHMIC SINE OF THE GIVEN ANGLE, AND FROM THE REMAINDER SUBSTRACT THE LOGARITHM OF THE GIVEN SIDE ; THE LAST REMAINDER WILL BE THE LOGARITHM OF THE SIDE REQUIRED.
OR, BY NATURAL SINES: DIVIDE THE DOUBLE AREA BY THE NAT. SINE OF THE GIVEN ANGLE, AND THAT QUOTI. ENT BY THE GIVEN SIDE; THE LAST QUOTIENT WILL BE THE SIDE REQUIRED.
Other methods of surveying fields are taught by some authors on this subject. The preceding, however, will be found most useful in actual practice. Other instruments besides those mentioned in this book are sometimes used; such as the plain table, semicircle, perambulator, theodolite, &c. But of these instruments very little use is made in New England; and they are not often to be met with. For gene. ral practice, none will be found more useful than a common chain, and a compass upon Rittenhouse's construction.
It will be observed that in this work there is no descrip. tions of mathematical and surveying instruments.
The compiler omitted such descriptions, from a belief that nothing which can be written on the subject will enable a person to understand them, without an actual inspection of the instru. ments themselves, and some instruction from those acquainted with them.
The general principles here taught may be applied to the surveying of townships, roads, rivers, harbours, &c.
OF THE DECLINATION AND VARIATION OF THE MAGNETIC
NEEDLE, AND OF THE ATTRACTIONS TO WHICH IT IS SUBJECT.
The declination of the needle is the number of degrees it deviates from the true north, either east or west. This differs in different places, and in the same place at different times. [At Hartford, Conn., the declination was, in 1829, 69 3' west of the true meridian of the earth; and was increasing by an annual variation of about 3'.]
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, 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 can therefore be due north only 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, at the latter period, because its motion be
ing then nearly vertical for some time, gives the observer opportunity to complete his observation.
To find the elongation of this star in any latitude, its de clination 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; 8o is co-sine of the declination; to sine of the elongation.
The declination of the north star, January 1, 1810, was 88° 17' 28", and increasing at the rate of about 19 seconds and one half annually.
In the following table, the elongation is calculated for ten successive years, ending with 1840, and for seven different latitudes. The calculation is made for the first of July, and of course gives the mean angle for the year.
1831 1° 521'|1° 55' 10 581 2° 11' 2° 5' 2° 9' 2° 13' 1832 1 52 1 541 1 58 2 1 2
81 2 124 18331 511 1 54 11 574 2
8 2 127 18341 51 1 531 1.57 1 591 2 4 2 71 2 12 1835 1 501 1 53 1 561 1 59 2 3 2 61 2 11 18361 50 1 524 1 56 1 581 2 21 |2
2 101 183711 5011 521 1 551 1 58.3 2 2 2 51 2 10 1838 1 50 1 521 1 55 1 58 2 11 2 5 2 1839 1 494 1 52 1 541 1 571 2 1 2 41 2 9 18401 49 1 511 1.54 1 571 2 01 2 4 2 84
* The following figure exhibits a view of tne relative situation of these stars, as they appear, when in a horizontal position; or when the north star is in its greatest eastern elongation.
The Great Bear.