A plane rectilineal angle is the inclination of two straight lines to one another in a plane which meet together, but are not in the same straight line as in Fig. 34.

When a straight line, standing on another 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 as in Fig 35.

Fig. 36.

An obtuse angle is that which is greater than a right angle as in Fig. 36.

An acute angle is that which is less than a right angle as in Fig. 34.

A term or boundary is the extremity of anything.

An equilateral triangle is that which has three equal sides as in Fig. 37.

An isosceles triangle is that which has two sides equal as in Fig. 38.

Fig. 37.

Fig. 38.

A scalene triangle is that which has three unequal sides as in Fig. 39.

A right angled triangle is that which has a right angle as in Fig. 40.

An obtuse-angled triangle is that which has an obtuse angle as in Fig. 39.

The hypothenuse in a right-angled triangle is the side opposite the right angle as in Fig. 40.

A square is that which has all its sides equal and all its angles right-angled as in Fig. 41.

A rectangle is that which has all its angles right angles, but only its opposite sides equal as in Fig. 42.

A rhombus is that which has all its sides equal, but its angles are not right angles as in Fig. 43.

A quadrilateral figure which has its opposite sides parallel is called a parallelogram as in Figs. 41, 42 and 43.

A line joining two opposite angles of a quadrilateral is called a diagonal.

Fig. 43.

Fig. 44.

An ellipse is a plane figure bounded by one continuous curve described about two points, so that the sum of the distances from every point in the curve to the two foci may be always the same-Fig. 44.

PROPERTIES OF THE CIRCLE.

A circle contains a greater area than any other plane figure bounded by the same length of circumference or outline.

A circle is a plane figure contained by one line and is such that all straight lines drawn from a point within the figure to the circumference are equal, and this point is called the center of the circle.

A diameter of a circle is a straight line drawn through the center and terminated both ways by the circumference, as AC in Fig. 45.

A radius is a straight line drawn from the center to the circumference, as LH in Fig. 45.

A semicircle is the figure contained by a diameter and that part of the circumference cut off by a diameter as AHC in Fig. 45.

A segment of a circle is the figure contained by a straight line and the circumference which it cuts off, as DHE in Fig. 45.

A sector of a circle is the figure contained by two straight lines drawn from the center and the circumference between them, as LMC in Fig. 45.

A chord is a straight line, shorter than the diameter, lying within the circle, and terminated at both ends by the circumference as DE in Fig. 45.

An arc of a circle is any part of the circumference as DHE in Fig. 45.

The versed sine is a perpendicular joining the middle of the chord and circumference, as GH in Fig. 45.

Fig. 46.

Circumference. Multiply the diameter by 3.1416, the product is the circumference.

Diameter. Multiply the circumference by .31831, the product is the diameter, or multiply the square root of the area by 1.12837, the product is the diameter. Area. Multiply the square of the diameter by .7854, the product is the area.

Side of the square. Multiply the diameter by .8862, the product is the side of a square of equal area.