The shaft, rectangular in cross-section, and beautifully cut through the solid sandstone rock, measured roughly 1½ metres by ¾ of a metre, tapering gradually downwards. The first few metres, sunk through the down-wash forming the bed of a valley, were carefully built in with blocks of limestone, the bottom layer resting on the solid rock below. Rough footholds on opposite sides of the shaft facilitate the descent, and when nearing the bottom, as the rope stretched and contracted like elastic in a most unpleasant manner, I was glad to avail myself of their assistance. The shaft, which I afterwards found to have a depth of 40·3 metres (132 feet), goes down perpendicularly, and when the bottom was reached, I found myself standing in a gently flowing stream of water, which I knew continued its underground course for at least 2 kilometres to the south.
The tunnel itself is distinctly coffin-shaped in cross-section, being widest near the roof, and tapering downwards. Its average height is about 1½ metres, the mean width near the top being about 60 centimetres—i.e., it measures roughly 5 feet by 2 feet. In some places it is even narrower, so that a man of average stature not only has to keep his head very much bent, but is also forced to progress sideways if he wishes to make his way along the channel. The place was so sultry, and the cramped position I had to assume so tiring, that I had soon had enough, and groped my way back to the base of the shaft. As I had anticipated, the ascent proved considerably more difficult than the descent, and I was not sorry when I regained the surface. After the heat of the shaft and the exertion of climbing, the outside air, despite the fact that it was an exceptionally hot day, felt bitterly cold, and the rapid evaporation from our perspiring bodies chilled us to the marrow.
Although I had vowed at the time that nothing would induce me to again enter the place, I afterwards reflected that, not having followed the tunnel to its extreme limits, there might still be further information to be gained, and at the termination there might exist an inscription, the deciphering of which would yield the much-desired information as to when and by whom the work had been carried out. Moreover, I particularly wished to ascertain whether the bulk of the water came from one particular point or whether it represented the accumulated flow from the numerous small fissures which a tunnel of such length must necessarily traverse. I therefore took the first opportunity of revisiting the place, this time providing myself with proper gear to facilitate the descent, and with instruments to make a thorough survey of the whole network of underground waterways.
The accompanying sketch-map, reduced from the detailed plan made during my last visit, shows that there are four main aqueducts, running nearly parallel, in a north and south direction, along the sides of three separate valleys. It is, of course, quite evident that the tunnels were run under the valleys rather than beneath the intervening ridges with the express object of keeping as near the surface as possible, so as to avoid unnecessary excavation in the sinking of the vertical shafts, and to reduce the labour involved in hoisting the excavated material to the surface. The engineers were at the same time careful to avoid the actual beds of the valleys, as there the soft nature of the ground would have necessitated a considerable amount of stone pitching, without which there would have been constant falls of the loose detritus, consisting of sand and pebbles with large blocks of limestone, forming the actual floors of the valleys. Alignments were chosen along the extreme margins, the mouths of the man-holes being commenced on the sloping sides of the valleys, a metre or two above their pebbly floors. The great majority were thus excavated throughout in solid rock, the latter being, as a rule, sufficiently firm to stand without timbering or masonry supports.
Although only one of the four main aqueducts is open at the present time, the exact course which each follows can be seen from the dump-heaps marking the positions of the shafts. The longest of the tunnels is the most westerly, measuring 4·6 kilometres from the point of origin to its exit on the west side of the ruined fort, the four together having a total length of 14·3 kilometres. The actual length of horizontal excavation is, however, considerably in excess of this figure, as there are very numerous subsidiary collecting branches ramifying from the main tunnels. Moreover, the total length given is the figure obtained by measuring along the surface from shaft to shaft; whereas, one of the most striking things underground is the markedly irregular trend of the tunnel, which frequently takes a wide bend between two adjacent shafts.
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THE SUBTERRANEAN AQUEDUCTS
OF
UM EL DABADIB
The man-holes, or vertical shafts connecting the underground waterway with the surface, served a double purpose. Through them the excavated material was hoisted to the surface, and in addition they afforded a means of ventilation, without which it is doubtful whether the work could have been carried out. The excavation of the man-holes alone must have been a gigantic task, equal to, if not greater than, that of cutting out the horizontal tunnels. The cross-section of an average-sized shaft is, if anything, greater than that of the tunnel; and although in vertical dimension the man-holes near the mouth of the aqueduct are insignificant, the depth rapidly increases in the opposite direction, owing to the upward slope of the ground-surface towards the escarpment. As already mentioned, the particular shaft I descended had a depth of over 40 metres, and subsequent levelling, from the exit of the aqueduct to a point above its origin, showed the most northerly one to have a depth of not less than 53½ metres (175 feet). Along this tunnel, which has a total length of 2·9 kilometres, I counted exactly 150 shafts, so that their average distance apart is between 19 and 20 metres.
That the tunnel was most carefully planned and excavated is evident from its very low and gradual slope, as is indicated by the depth of water flowing through and by the generally unrippled character of the stream. Judging by the height of the mouth of one of the man-holes near the point of origin of the tunnel, and by its depth to the stream in the tunnel below, there only appears to be a fall of 1 metre in about 2½ kilometres, or a slope of 1 in 2,670. It is possible that, owing to the elasticity of the line used, there may be a slight error in the measurement of the shaft, but the above figure may be taken as approximately correct.