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SECTION IIL.

MENSURATION OF AREAS, OR THE VARIOUS METHODS OF CALCULATING THE SUPERFICIAL

CONTENT OF ANY FIELD.

DEFINITION. THE area or content of any plane surface, in perches, is the number of square perches which that surface contains.

PL. 7. fig. 1. Let ABCD represent a rectangular parallelogram, or oblong : let the side AB, or DC, contain 8 equal parts; and the side AD, or BC, three of such parts ; let the line AB be moved in the direction of AD, tillit has come to EF; where AE, or BF (the distance of it from its first situation) may be equal to one of the equal parts. Here it is evident, that the generated oblong ABEF will contain as many squares as the side AB contains equal parts, which are 8; each square have ing for its side one of the equal parts, into which AB, or AD, is divided. Again, let AB move on till it comes to GH, so as GE, or HF, may be equal to AE, or BF; then it is plain that the oblong AGHB, will contain twice as many squares as the side AB contains equal parts. After the same mannerit willappear, that the oblong ADCB will contain three times as many squares as the side AB contains equal parts; and in general, that every rectangular parallelogram, whether square or oblong, contains as many squares as the product of the number of equal parts in the base, multiplied into the number of the same equal parts in the height, contains units, each square having for its side one of the equal parts.

Hence arises the solution of the following problems.

PROB. I.

To find the content of a square piece of ground.

1. Multiply the base in perches, into the perpendicular in perches, the product will be the content in perches; and because 160 perches make an acre, it must thence follow, that

Any area, or content in perches, being divided by 160, will give the content in acres; the remaining perches, if more than 40, being divided by 40, will give the roods, and the last remainder, if any, will be perches.

Or thus :

2. Square the side in four-pole chains and links, and the product will be square four-pole chains and links: divide this by 10, or cut off one more than the decimals, which are five in all, from the right towards the left: the figures on the left are acres; because 10 square four-pole chains make an acre, and the remaining figures on the right, are decimal parts of an acre. Multiply the five figures to the right by 4, cutting 5 figures from the product, and if any figure be to the left of them, it is a rood, or roods; multiply the last cut off figures by 40, cutting off five, or (which is the same thing) by 4, cutting off four; and the remaining figures to the left, if any, are perches.

1. The first part is plain, from considering that a piece of ground in a square forin, whose side is a perch, must contain a perch of ground; and that 40 such perches make a rood, and four roods an

Dd

acre ; or which is the same thing, that 160 square perches make an acre, as before.

2. A square four-pole chain (that is, a piece of ground four poles or perches every way) must contain 160 square perches; and 160 perches make an acre, therefore 10 times 16 perches, or 10 square four-pole chains, make an acre.

Note. The chains given, or required, in any of the following problems, are supposed to be twopole chains, that chain being most commonly used; but they must be reduced to four-pole chains or perches for calculation, because the links will not operate with them as decimals.

EXAMPLÉS.

PL. l. fig. 17.

Ch. L. Let ABCD be a square field, whose side is 14 29, required the content in acres.

Cl. D.
By problem 4. section 1. part 2. 14. 29 are equal to

29.16 perches
29.16

17496

2916 26244 5832

160)850.3056

A. R. P.
5. 1. 10. content.

40)50(1 rood.

10 perches.

Or thus :

Ch. L.

Ch. L. 14. 29 are equal to 7. 29 of four-pole chains, by prob. 1. sect. 1. pt. 2. 7. 29

6561 1458 5103

A.R. P. Acres 5|31441 cont. as before 5. 1. 10

4

Rood 1(25764

40

Perches 10/30560

It is required to lay down a map of this piece of ground, by a scale of twenty perches to an inch.

Take 29. 16 the perches of the given side, from the small diagonal on the common surveying scale, where 20 small, or ttvo of the large divisions, are an inch: make a square whose side is that length (by prob. 9. geoin.) and it is done.

PROB. II.

To find the side of a square, whose content is given.

Extract the square root of the given coxtent in perches, and you have the side in perches, and consequently in chains.

EXAMPLE

It is required to lay out a square piece of ground which shall contain 12A. 3R. 16P. Required the number of chains in each side of the square; and to lay down a map of it, by a scale of 40 perches to an inch,

A. R. P.
12.

3. 16.
4

51 40

Ch. L. 2056(45.34+ perches = 22. 33 by prob. 6.

[blocks in formation]

From a scale where 4 of the large, or 40 of the small divisions are an inch, take 45.34, the perches of the side, of which make a square.

PROB, III.

To find the content of an oblong piece of ground.

Multiply the length by the breadth, for the content.

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