E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Line of Direction.
The line in which a body moves, a force acts, or motion is communicated. In order that a body may stand without falling, a line let down from the centre of gravity must fall within the base on which the object stands. Thus the leaning tower of Pisa does not fall, because this rule is preserved.