XV.

A circle is a plain figure contained by one line, which is called the circumference, and is such that all straight lines drawn from a certain point within the figure to the circumference, are equal to one another.

Book I.

XVI.

XIX

N

And this point is called the center of the circle.

XVII.

A diameter of a circle is a straight line drawn thro' the center, See N.

and terminated both ways by the circumference.

XVIII.

A femicircle is the figure contained by a diameter and the part of

the circumference cut off by the diameter.

XIX.

" A fegment of a circle is the figure contained by a straight line

" and the circumference it cuts off."

XX.

Rectilineal figures are those which are contained by straight

lines.

XXI.

Trilateral figures, or triangles, by three straight lines.

XXII.

Quadrilateral, by four straight lines.

XXIII.

Multilateral figures, or Polygons, by more than four straight lines.

XXIV.

Of three fided figures, an equilateral triangle is that which has three equal fides.

XXV.

An isosceles triangle, is that which has only two fides equal.

Book I.

AAA

XXIV

XXVI.

XXVI

A fcalene triangle, is that which has three unequal fides.

XXVII.

A right angled triangle, is that which has a right angle.

XXVIII.

An obtuse angled triangle, is that which has an obtuse angle.

See N.

An acute angled triangle, is that which has three acute angles.

XXX.

Of four fided figures, a square is that which has all its fides equal, and all its angles right angles.

An oblong is that which has all its angles right angles, but has not all its fides equal.

XXXII.

A rhombus is that which has all its fides equal, but its angles are not right angles.

XXXIII.

A rhomboid is that which has its opposite sides equal to one another, but all its fides are not equal, nor its angles right angles.

XXXIV.

All other four sided figures besides these, are called Trapeziums.

XXXV.

Parallel straight lines are such as are in the same plane, and which, being produced ever so far both ways, do not meet.

L

POSTULATES.

I.

ET it be granted that a straight line may be drawn from
any one point to any other point.

• II.

That a terminated straight line may be produced to any length in a straight line.

III.

And that a circle may be described from any center, at any distance from that center.

ΑΧΙΟMS.

I.

HINGS which are equal to the same are equal to one

T

another.

II.

If equals be added to equals, the wholes are equal.

III.

If equals be taken from equals, the remainders are equal. X

IV.

If equals be added to unequals, the wholes are unequal.

V.

If equals be taken from unequals, the remainders are unequal.

VI.

Things which are double of the fame, are equal to one another.

VII.

Things which are halves of the same, are equal to one another.

VIII.

Magnitudes, which coincide with one another, that is, which exactly fill the same space, are equal to one another.

Book I.

"If a straight line meets two straight lines, so as to make the " two interior angles on the same side of it taken together less " than two right angles, these straight lines being continually " produced shall at length meet upon that fide on which are "the angles which are less than two right angles. See the "notes on Prop: 29. of Book I."

$

PROPOSITION I. PROBLEM.

O describe an equilateral triangle upon a given

To defcriben qui

Let AB be the given straight line, it is required to describe an equilateral triangle upon it.

Because the point A is the center of the circle BCD, AC is equal • to AB. and because the point B is the center of the circle ACE, c. 15th DeBC is equal to BA. but it has been proved that CA is equal to AB; therefore CA, CB are each of them equal to AB. but things which are equal to the fame are equal to one another d; therefore d. ist AxiCA is equal to CB. wherefore CA, AB, BC are equal to one another. and the triangle ABC is therefore equilateral, and it is defcribed upon the given straight line AB. Which was required to be done.

finition.

om.

F

ROM a

given point to draw a straight line equal

to a given straight line.

Let A be the given point, and BC the given straight line; it is required to draw from the point A a straight line equal to BC. From the point A to B draw a the

straight line AB; and upon it describe the equilateral triangle D AB, and produce the straight lines DA, DB to E and F; from the center B, at the distance BC defcribed the circle CGH, and from the center D, at the distance DG described the circle GKL. AL thall be equal to

BC.

C

K

H
DA

B

G

F

a. 1. Poft.

b. I. I.

c. 2. Poft.

d. 3. Pofta