The Elements of Perspective / arranged for the use of schools and intended to be read in connection with the first three books of Euclid

by John Ruskin

will be the line required.

I shall hereafter call the line A C, when used to define the position of an inclined line A B ([Fig. 40.]), the “relative horizontal” of the line A B.

Observation.

Fig. 43.

In general, inclined lines are most needed for gable roofs, in which, when the conditions are properly stated, the vertical height of the gable, X Y, [Fig. 43.], is given, and the base line, A C, in position. When these are given, draw A C; raise vertical A D; make A D equal to sight-magnitude of X Y;

complete the perspective-rectangle A D B C; join A B and D C (as by dotted lines in figure); and through the intersection of the dotted lines draw vertical X Y, cutting D B in Y. Join A Y, C Y; and these lines are the sides of the gable. If [p52] ]the length of the roof A A′ is also given, draw in perspective the complete parallelopiped

A′ D′ B C, and from Y draw Y Y′ to the vanishing-point of A A′, cutting D′ B′ in Y′. Join A′ Y, and you have the slope of the farther side of the roof.

Fig. 44.