When, after much mean and yet costly endeavour to patch up the cathedral
of St. Paul's, after the great fire, Sir Christopher Wren at last had
his advice accepted, to rebuild the whole structure, the demolition of
the old fabric gave ample play to his scientific knowledge and
engineering skill. One of his exploits, perhaps now more remarkable
because at the time it was at once rare and bold, has thus been
described:--"In order that the rubbish and old materials might not
hinder the setting out of the foundations, for the purpose of proceeding
with the works, Sir Christopher constructed scaffolds high enough to
extend his lines over the heaps that were in the way; and thereby caused
perpendiculars to be fixed upon the points below for his various walls
and piers, from lines drawn carefully upon the level plan of the
scaffold. Thus he proceeded, gaining every day more and more room, till
he came to the middle tower that formerly carried the lofty spire. The
ruins of this tower being nearly two hundred feet high, the labourers
were afraid to work above, which induced him to facilitate the labour by
the use of gunpowder. To perform this work, he caused a hole to be dug,
of about four feet wide, by the side of the north-west pier of the
tower, in which was perforated a hole two feet square, reaching to the
centre of the pier. In this he placed a small deal box containing
eighteen pounds of gunpowder. To this box he affixed a hollow cane,
which contained a quick match, reaching to the surface of the ground
above; and along the ground a train of powder was laid, with a match.
The mine was then closed up, and exploded, while the philosophical
architect waited with confidence the result of his experiment. This
small quantity of powder not only lifted up the whole angle of the
tower, with two great arches that rested upon it, but also two adjoining
arches of the aisle, and the masonry above them. This it appeared to do
in a slow but efficient manner, cracking the walls to the top, lifting
visibly the whole weight about nine inches, which suddenly dropping,
made a great heap of ruins in the place, without scattering or accident.
It was half a minute before the heap already fallen opened in two or
three places, and emitted smoke. By this successful experiment, the
force of gunpowder may be ascertained; eighteen pounds only of which
lifted up a weight of more than three thousand tons, and saved the work
of a thousand labourers. The fall of so great a weight from a height of
two hundred feet gave such a concussion to the ground, that the
inhabitants round about took it for the shock of an earthquake."
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