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RULE 2. If it be a Right Angled Triangle, multiply one of the Legs into half the other ; the Product will be the Area. Or, multiply the two Legs into each other, and half the Product will be the Area.
RULE 3. When the three sides of a Triangle are known, the Area may be found Arithmetically, as follows :
Add together the three Sides; from half their Sum subtract each side, noting down the Remainders ; multiply the half Sum by one of those Remainders, and that Product by another Remainder, and that Product by the other Remainder; the Square Root of the last Product will be the Area.
EXAMPLE. Suppose a Tringle whose three Sides are 24, 20 and 18 Chains. Demanded the Area.
24+20+18=62, the Sum of the three Sides, the half of which is 31. From 31 subtract 24, 20 and 18; the three Remainders will be 7, 11 and 13.
31X7=217; 217x11=2387 ; 2387x13=31031, the Square Root of which is 176.1 or 17 Acres 2 Roods ard 17 Rods.
By Logarithms. As the Addition of Logarithms is the same as the Multiplication of their corresponding Numbers; and as the Number answering to the one half of a Logarithm will be the Square Root of the Number corresponding to that Logarithm ; it follows, That if the Logarithm of the half
Sum of the three Sides and the Logarithms of the three Remainders be added together, the Num. ber corresponding to one half the Sum of those Loga rithms will be the Area of the Triangle. The half Sum, 31
1.49136 The first Remainder, 7
0.84510 The second Remainder, 11
1.04139 The third Remainder, 13
The Square of the Area, 31000
Rule 4. When two sides of a Triangle and their contained Angle, that is, the Angle made by those Sides, are given, the Area may be found as follows:
Add together the Logarithms of the two Sides and the Logarithmic Sine of the Angle; from their Sum subtract the Logarithm of Radius, the Remainder will be the Logarithm of double the Area.
EXAMPLE. Suppose a Triangle one of whose Sides is 105 Rods and another 85 and the Angle contained between them 28° 5'. Demanded the Area. One Side, 105
2.02119 The other Side, 85
1.92942 Sine Angle, 28° 5'
Double Area, 4200 Rods
Answer. 2100 Rods.
figure of the Index, or subtracting 10, without the trouble of
By Natural Sines. Multiply the two given Sides into each other, and that Product by the Natural Sine of the given Angle ; the last Product will be double the Area of the Tri. angle.
Nat. Sine of the Angle 28° 5'. 0.47076
105X85=8925, and 8925X0.47076=4201 the double Area of the Triangle.
PROBLEM X. To find the Area of a Trapezoid.
RULE. Multiply half the Sum of the two parallel Sides by the perpendicular distance between them, or the Sum of the two parallel Sides by half the perpendicular distance; the Product will be the Area.
PROBLEM XI. Toʻfind the Area of a Trapezium, or irregular Four Sided Figure.
Rule. Draw a Diagonal between two opposite
Triangles. Find the Area of each Triangle and add them together. Or, multiply the Diagonal by half the Sum of the two Perpendiculars let fall upon it, or the Sum of the two Perpendiculars by half the Diagonal ; the Product will be the Area. Note. Where the length of the four Sides and of the Diagonal
is known, the Area of the two Triangles, into which the Trapezium
is divided, may be calculated Arithmetically, accord ing to PROB. IX. Rule 3. PROBLEM XII. To find the Area of a Figure containing more than Four Sides.
Rule. Divide the Figure into Triangles and Trapezia, by drawing as many Diagonals as are necessary; which Diagonals must be so drawn as not to intersect each other : Then find the Area of each of the several Triangles or Trapezia, and add them together ; the Sum will be the Area of the whole Figure. Notc. A little practice will suggest the most convenient way of
drawing the Diagonals ; but whichever way they are drawn, provided they do not intersect each other, the whole Area will be found the same. PROBLEM XIII. Respecting Circles.
Rule 1. If the Diameter be given, the Circumference may be found by one of the following Proportions : As 7 is to 22; or more exactly, as 113 is to 355; or in Decimals, as 1 is to 3.14159 ; so is the Di. ameter to the Circumference.
Rule 2. If the Circumference be given, the Diameter may be found by one of the following Proportions : As 22 is to 7; or as 355 is to 113; or as 1 is to 0.31831; so is the Circumference to the Diameter:
RULE 3. The Diameter and Circumference being known, multiply half the one into half the other, and the Product will be the Area.
RuļE. 4. From the Diameter only to find the Area: Multiply the Square of the Diameter by 0.7854, and the Product will be the Area.
RULE 5. From the Circumference only to find the Area : Multiply the Square of the Circumference by
Rule 6. The Area being given to find the Diam. eter: Divide the Area by 0.7854, and the Quotient will be the Square of the Diameter; from this extract the Square Root, and you will have the Diameter.
Rule 7. The Area being given to find the Circumference : Divide the Area by 0.07958, and the Quotient will be the Square of the Circumference; from this extract the Square Root, and you will have the Circumference.
The following Cases teach the most usual methods of taking the Survey of Fields ; also how to protract or draw a Plot of them, and to calculate their Area. Note. The FIELD Book is a Register containing the length of
the sides of a Field, as found by measuring them with a Chain ; also the Bearings or Courses of the Sides, or the Quantity of the several Angles, as found by a Compass, or other instrument for that purpose ; together with such Remarks as the Surveyor thinks proper to make in the Field.
CASE I. To survey a Triangular Field. Measure the Sides of the Field with a Chain, and enter their several lengths in a Field Book; protract the Field on Paper, and then find the Area by Prob. IX. Rule 1. Or, without plotting the Field, calculate the Area by Prob. IX. Rule 3. FIELD BOOK. Sce PLATE II. Fig. 46.
9600 7200 16800
Acres Roods Rods Area 17 - 2 18.56 Vote. When there are Cyphers at the Right Hand of the Links,
they may be rejected ; remembering to cut off a proper number of figures according to Decimal Rules.
Observe, That in measuring with a Chain, slant or inclined Surfaces, as the Sides of Hills, should be measured horizontally, and not on the Plane or Surface of the Hill; otherwise a Survey cannot be accurately taken. To effect this, the lower end of the Chain must be raised from the ground, so as to have the whole in a horizontal Line; and the end thus raised must be directly over the Point where the Chain begins or ends, according as you are ascending or descending a Hill ; which Point may be ascertained by a Pluminet and Line.