790

THEORY AND PRACTICE OF ENGINEERING.

Book II.

number of these rays is called a pyramid: to find the perspective of the small house inthe accompanying figure, it is only necessary to find the points of intersection of the visualrays, made with the visible angles of the object as they pierce the plane which cuts them.

The point of view or point of sight is the eye of the observer.

The plane of delineation or picture is the canvass or paper upon which the subject isdrawn.

The original object is the house or any other to be drawn.

The original lines are the boundaries of the original objects, or of planes in those objects.

The horizontal planes are any planes placed truly level, that is, to which the plumb lineis perpendicular.

The horizontal line is a line on the plane of delineation level with the eye of the observer,or point of view, and is supposed to be obtained by a horizontal plane passing through theeye of the observer produced till it cuts the plane of delineation.

Vertical planes are planes perpendicular to horizontal planes.

Ground plane is that upon which the objects to be drawn are placed, and also thaton which the observer stands.

Ground line is that on which the plane of delineation rests on the ground line.

Station point is that on the ground plane perpendicularly under the point of sight orpoint of view.

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Fig. 905.

Vanishing point is a point on the plane of delineation, which is the point of union or