The air, though cold, could be breathed easily, and the lights did not grow dim in it as they do in subterranean places where the atmosphere is foul.  The stream of water, flowing swiftly in its deep channel from under the little arch, brought plentiful ventilation into it.  Above, there was no aperture in the vaulting, but there was one in the mediaeval masonry that projected into the chamber.  There, on the side towards the right, where the water flowed in, Malipieri had found a narrow slit, barely wide enough to admit a man’s open hand and wrist, but nearly five feet high, evidently a passage intended for letting the water flow into the interior of the construction when it overflowed its channel and rose above the floor of the chamber.

At first Malipieri had supposed that this aperture communicated with some ancient and long-forgotten drain by which the water could escape to the Tiber; it was not until he had gained an entrance to the hollow mass of masonry that he understood the hideous use to which it had been applied.

It had not been hard to enlarge it.  Any one who has worked among ruins in Italy could tell, even blindfold, the difference between the work done in ancient times and that of the middle ages.  Roman brickwork is quite as compact as solid sandstone, but mediaeval masonry was almost invariably built in a hurry by bad workmen, of all sorts of fragments embedded in poorly mingled cement, and it breaks up with tolerable ease under a heavy pickaxe.

In half a day Malipieri and Masin had widened the slit to a convenient passage, but as soon as it had been possible to squeeze through, the architect had gone in.  He never forgot what he felt when he first looked about him.  Masin could not follow him until many blows of the pick had widened the way for his bulkier frame.

Malipieri stopped at the entrance now, holding his lantern close to the ground, and looking for traces of footsteps.  He found none, but as he was about to move forward he uttered an exclamation of surprise, and picked up a tiny object which he held close to the light.  It was only a wax match, of which the head had been broken off when it had been struck, so that it had not been lighted.  That was all, but neither he nor Masin carried wax matches in the vaults, because the dampness soon made them useless.  They took common sulphur matches in tin match-boxes.  Besides, this was an English wax light, as any one could tell at a glance, for it was thicker, and stiffer, and longer than the cheaper Italian ones.

Malipieri drew back and showed it to his man, who examined it, understood, and put it into his pocket without a word.  Then they both went in through the aperture in the wall.

The masonry outside was rectangular, as far as it could be seen.  Inside, it was built like a small circular cistern, smoothly cemented, and contracting above in a dome, that opened by a square hole to the well-shaft above.  Like the stones in the outer chamber, the cement was coated with scales of dried mud.  The shaft was now certainly closed at the top, for in the daytime not a ray of light penetrated into its blackness.