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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. To find the area of a trapezium, or irregular four sided figure.
Rule. Draw a diagonal between two opposite angles, which will divide the trapezium into two 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, according 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. Note. 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 I is to 3.14159, so is the diameter 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.
RULE 4. From the diameter only, to find the area : mul. tiply the square of the diameter by 0.7854, and the product will be the area.
multiply the square of the circumference by 0.07958, and the product will be the area.
RULE 6. The area being given, to find the diameter: 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.
To find the area of an Ellipse or an Ellipsis
RULE. Multiply the product of the two diameters by 0.7854, and point off from the product as many figures at the right hand as there are decimals in both factors, and those at the left will be the area of the figure.
EXAMPLE. What is the area of an ellipse, of which the longest diameter is 65 rods and 10 links, and the shortest is 45 rods and 10 links.
Longest diameter, 65.4
2616 3270 2616
1187664 1484580 2375328 2078412
92 A. Q. R.
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.
TO SURVEY A TRIANGULAR FIELD.
Measure the sides of a 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.
18 To find the area.
Ch. L. Base B C
24.00 Half perp. A D 7.34 A
B 20 Ch.
9600 7200 16800
Acres Roods Rods Area 17-2-18.56 5
NOTE. When there are ciphers 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 plummet and line.
TO SURVEY A FIELD IN THE FORM OF A TRAPEZIUM.
Measure the several sides, and a diagonal between two opposite angles; protract the field, and find the area by PROBLEM XI. Or, without protracting the field, calculate the area according to the note at the end of that PROBLEM.
14.70 Diag. 'AC 28.
TO PROTRACT THIS TRAPEZIUM,
Draw the side A B, the given length; with the diagonal AC 28, and the side B C 11.70, describe cross arcs as at C, from A and B as centres; and the point of intersection will represent that corner of the field : then, with the side CD 21.50, and the side A D 14.70, describe cross arcs as at D, from A and C as centres; and the point of intersection will repre
Note. The perpendiculars need not be actually drawn; their length
may be obtained as follows: from the angle opposite the diagonal open the dividers so as when one foot is in the angular point, as at B, the other, being moved backwards and forwards, may just touch the diagonal at A, and neither go the least above or below it; that distance in the dividers being measured on the scale, will give the length of the perpendicular.
TO SURVEY A FIELD WHICH HAS MORE THAN FOUR SIDES,
BY THE CHAIN ONLY.
Measure the several sides, and from some one of the angles from which the others may be seen, measure diagonals to them; draw a plot of the field, and find the area by PROB