Whittock, Nathaniel: The Oxford Drawing Book, Or The Art Of Drawing, And The Theory And Practice Of Perspective: In A Series Of Letters Containing Progressive Information On Sketching, Drawing, And Colouring Landscape Scenery, Animals, And The Human Figure: With A New Method Of Practical Perspective: Detailed In A Novel, Easy, And Perspicuous Style, For The Use Of Teachers, Or For Self-Instruction. Embellished With Upwards Of One Hundred And Fifty Lithographic Drawings, From Real Views, Taken Expressly For This Work (Oxford , London, 1825)

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high ; in order to shew you how a scale is formed, I have drawn the scale of ten feet at
the bottom of the drawing, by which you will measure the length of the wall on the
base line, and the height of it on the line of elevation: having done this, you must
draw lines from b and c to the point of sight, and from cle, the measure on the base,
to the point of distance; the point where the lines drawn from the base intersect that
drawn from b to the point of sight, is the true length of the wall in perspective ; and
if from the intersection fg, you draw perpendiculars to the upper line, it will give
the height of the wall. If you wish to give the thickness, which is here one foot and
a half, this must likewise be measured on the base line: we have now obtained the
height, length, and thickness of the wall in true perspective. The door-way in the
centre of the wall is six feet wide, and ten feet high. To ascertain on what part of
the wall this should be placed, find the centre of the admeasurement of the length of
the wall on the base line, and set off three feet on each side of it; this is shewn by
i k: from these points draw lines to the point of distance, and the points where
they intersect the line drawn from the bottom of the line of elevation to the point of
sight, shew the spot where the perpendicular lines are to be drawn for the sides of
the door-way. To obtain the true height, measure ten feet on the line of elevation,
and thence draw a line to the point of sight; the lines drawn before, will give you the
thickness of the wall at the opening.