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 column of Pisa to be in violation of this
principle; but, according to designs shown to Dr. Cumming, at Pisa, in
1836, the perpendicular does fall within the base.
-----
Footnote 2:
When at Pisa, many years since, Captain Basil Hall investigated the
origin and divergence of the tower from the perpendicular, and
established completely to his own satisfaction that it had been built
from top to bottom, originally, just as it now stands. His reasons for
thinking so are, that the line of the tower, on that side towards
which it leans, has not the same curvature as the line on the
opposite, or what may be called the upper side. If the tower had been
built upright, and then been made to incline over, the line of the
wall on that side towards which the inclination was given, would be
more or less concave in that direction, owing to the nodding or
"swagging over" of the top, by the simple action of gravity acting on
a very tall mass of masonry, which is more or less elastic when placed
in a sloping position. But the contrary is the fact; for the line of
wall on the side towards which the tower leans, is decidedly more
convex than the opposite side. Captain Hall has, therefore, no doubt
whatever that the architect, in rearing his successive courses of
stones, gained or stole a little at each layer, so as to render his
work less and less overhanging as he went up; and thus, without
betraying what he was about, really gained stability.
------------------