A POINT is that which hath no parts, or which hath no mag- See

nitude.

II.

A line is length without breadth.

III.

The extremities of a line are points.

IV.

Notes.

A straight line is that which lies evenly between its extreme points.

V.

A superficies is that which hath only length and breadth.

VI.

The extremities of a superficies are lines.

VII.

A plane superficies is that in which any two points being taken, See N.

the straight line between them lies wholly in that superficies.

VIII.

" A plane angle is the inclination of two lines to one another See N. " in a plane, which meet together, but are not in the same "direction."

IX.

A plane rectilineal angle is the inclination of two straight lines to one another, which meet together, but are not in the same straight line.

B

N. B. When several angles are at one point B, any one of ' them is expressed by three letters, of which the letter that is ' at the vertex of the angle, that is, at the point in which the ' straight lines that contain the angle meet one another, is put 'between the other two letters, and one of these two is some' where upon one of those straight lines, and the other upon 'the other line: thus the angle which is contained by the ' straight lines AB, CB is named the angle ABC, or CBA; that • which is contained by AB, DB is named the angle ADB, or Al

DBA; and that which is contained by DB, CB is called the 'angle DBC, or CBD; but, if there be only one angle at a ' point, it may be expressed by a letter placed at that point; as 'the angle at E.'

Χ.

When a straight line standing on ano-
ther straight line makes the adjacent
angles equal to one another, each of
the angles is called a right angle;
and the straight line which stands on
the other is called a perpendicular to
it.

XI.

An obtuse angle is that which is greater than a right angle.

XII.

An acute angle is that which is less than a right angle.

XIII.

"A term or boundary is the extremity of any thing."

XIV.

A figure is that which is enclosed by one or more boundaries.

XV.

A circle is a plane 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.

And this point is called the centre of the circle.

XVII.

A diameter of a circle is a straight line drawn through the centre, and terminated both ways by the circumference.

XVIII.

A semicircle is the figure contained by a diameter and the part of the circumference cut off by the diameter.

ΧΙΧ.

" A segment 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.

ΧΧΙ.

Trilateral figures, or triangles, by three straight lines.

ΧΧΙΙ.

Quadrilateral, by four straight lines.

ΧΧΙΙΙ.

Multilateral figures, or polygons, by more than four straight

lines.

XXIV.

Of three sided figures, an equilateral triangle is that which has three equal sides.

An isosceles triangle is that which has only two sides equal.

Book I.

XXVI.

A scalene triangle is that which has three unequal sides.

XXVII.

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

XXVIII.

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

ΧΧΙΧ.

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

xxx.

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

XXXI.

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

ΧΧΧΙΙ.

A rhombus is that which has all its sides 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 sides are not equal, nor its angles right angles.