Cox, David: A series of progressive lessons intended to elucidate the art of painting: with introductory illustrations on perspective and drawing with pencil (London, 1845)

SKETCHING.
Having fixed upon a subject suited to his powers, the pupil will commence
his sketch by faintly marking the lines which form the boundaries of the
principal masses in the picture, such as the leading lines of the foreground-
middle ground, and distance, in order that he may forecast the space which
each will require, and ascertain the situation of the most important points. In
doing this, he will aim at giving the general proportions of each sketch in the
various forms before he proceeds with their detail. Inconveniences will thus
be avoided, as he would sometimes discover disproportions in the arrangement,
and perhaps, also, a want of space which would oblige him to efface all that
had been done. In particular, he will mark the perspective horizon, that he
may, by comparing with the height of his own eye in the scene, give to all
the level surfaces their proper appearance of flatness, and secure, where
practicable, the situation of the vanishing point of any lines of buildings, &c.