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."