DEFINITIONS, ILLUSTRATIONS, ETC.

B A Point is that which has position, but not magnitude. c A Straight Line is that which lies evenly between the two points at its extremities.

DA Plane is a surface, real or imaginary, such, that any two points in it being taken and joined by a straight line, the straight line will lie altogether in that surface or plane.

FA 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.

That point within the circle which is thus equally distant from the circumference, is called the Centre.

HA Circle may be described from any centre, at any distance from that centre.

Illustration. The sensible horizon, or circular boundary, which every individual, standing in a clear space, beholds around him, where the sky appears to touch the earth, has the station of that observer for its centre; and this horizon will vary its position with every step he takes.

The magnitude likewise, of this circular boundary, varies as his eye is elevated or depressed; for instance, if the individual ascend a ship's mast, or look out of its cabin window, or swim with his head just above the surface of the water.

The distance of the centre from every part of the circumference is called the Radius.