Sir John Leslie used to attribute the stability of this tower to
the cohesion of the mortar it is built with being sufficient to
maintain it erect, in spite of its being out of the condition required
by physics--to wit, that “in order that a column shall stand, a
perpendicular let fall from the centre of gravity must fall within the
base.” Sir John describes the Tower of Pisa to be in violation of this
principle; but, according to later authorities, the perpendicular falls
within the base.