Unfortunately descent by any of these shafts proved out of the question in the time available, owing to their mouths having been carefully closed by great slabs of rock, which I had no facilities for moving. This discovery upset my calculations in two ways: unless I abandoned the undertaking altogether, which I was extremely loath to do after coming so far, it would be necessary to follow the tunnel from the bottom of the ninth man-hole for at least 230 metres, and I had already experienced the difficulty of progressing even a short distance in so confined a space. Secondly, there was the question of ventilation. With an open shaft near the top there would at least have been good circulation through the greater part of the aqueduct; however, I still hoped that the upper man-holes were not so tightly closed as to render the air below entirely stagnant. One of the natives who had taken part in the cleaning out of the aqueduct many years before asserted that the air was not altogether bad, and though I could not definitely ascertain whether or no he had been to the end of the tunnel since the closing of so many of the ventilating shafts, I determined to accept his word for it, and we descended safely to the bottom. The water felt cool by comparison with the air of the tunnel, though my thermometer showed the temperature of both air and water to be identical, registering 87° F. in both cases. It was quite sweet, though a good deal of sediment appeared when the stream was stirred up. After making a number of measurements of the shaft and tunnel, we proceeded on our way upstream. The sides of the tunnel were everywhere beautifully cut in a light brown sandstone, the direction of the pick-marks showing that it had been excavated in the same direction as that we were following—_i.e._, from south to north. Small notches for the lights used by the workmen were plainly observable on the walls, about half-way between the floor and the roof. As a rule the tunnel maintains an elongated oval form, wider near the top than elsewhere; along one length, however, where it evidently follows a fissure, both top and bottom taper sharply, so much so that our feet constantly got jammed in the narrow, wedge-shaped channel through which the water flows. We passed many side branches, blind alleys, in fact, and the main tunnel itself zigzagged considerably, frequently turning at sharp angles, and more than once almost doubling back on itself.