Ere long, however, his emotions took a sterner and a darker hue. The sight of the animal wearied him, and he flung it contemptuously aside. It disappeared in the direction of the ramparts; and almost at the same moment he heard a slight sound, resembling the falling of several minute particles of brick or light stone, which seemed to come from the wall behind him. That such a noise should proceed from so massive a structure appeared unaccountable. He rose, and, parting the bushes before him, advanced close to the surface of the lofty wall. To his astonishment, he found that the brickwork had in many places so completely mouldered away, that he could move it easily with his fingers. The cause of the trifling noise that he had heard was now fully explained: hundreds of lizards had made their homes between the fissures of the bricks; the animal that he had permitted to escape had taken refuge in one of these cavities, and in the hurry of its flight had detached several of the loose crumbling fragments that surrounded its hiding-place. Not content, however, with the discovery he had already made, he retired a little, and, looking stedfastly up through some trees which in this particular place grew at the foot of the wall, he saw that its surface was pierced in many places by great irregular rifts, some of which extended nearly to its whole height. In addition to this, he perceived that the mass of the structure at one particular point, leaned considerably out of the perpendicular. Astounded at what he beheld, he took a stick from the ground, and inserting it in one of the lowest and smallest of the cracks, easily succeeded in forcing it entirely into the wall, part of which seemed to be hollow, and part composed of the same rotten brickwork which had at first attracted his attention. It was now evident that the whole structure, over a breadth of several yards, had been either weakly and carelessly built, or had at some former period suffered a sudden and violent shock. He left the stick in the wall to mark the place; and was about to retire, when he heard the footstep of the sentinel on the rampart immediately above. Suddenly cautious, though from what motive he would have been at that moment hardly able to explain, he remained in the concealment of the trees and bushes, until the guard had passed onward; then he cautiously emerged from the place; and, retiring to some distance, fell into a train of earnest and absorbing thought. To account to the reader for the phenomenon which now engrossed the Pagan's attention, it will be necessary to make a brief digression to the history of the walls of Rome.