The Aswân Obelisk / With some remarks on the Ancient Engineering

by Reginald Engelbach

(21) Having examined, as far as possible, the methods by which the obelisk was separated from the surrounding rock, we will consider by what means the obelisk was detached from its bed and got into a position in which it could be handled and transported.

It might be remarked that this particular obelisk has not been transported; there is no doubt, however, that the man responsible for the work had quite definite ideas as to how he was going to perform the feat. Although it is the largest obelisk known (pace the phantom 108 cubit obelisk of

atshepsôwet), the old engineers have actually moved even heavier and more unmanageable blocks: the colossus of the Ramesseum and the colossi of Amenophis III at Thebes. If we can solve the ancient method of dealing with this particular obelisk, we can the more easily understand how the others were dealt with.

There seem to be two methods by which the obelisk could be detached from its bed; the snapping off of such an obelisk in the manner mentioned in section [19] being out of the question.

(1) By undercutting the obelisk from both sides to a certain extent, say a quarter of the breadth from each side, and either detaching it by a series of very large wedge-slots (as was done all over the quarries for medium-sized blocks), or, if the Egyptians used wooden wedges, expanded by the action of water, by one long wedge channel on each side of the obelisk. These could be wetted by flooding the trench with water, but before this could be done, the trench would have to be divided into compartments by, say, mud-brick walls to prevent the water running down to the deep end, leaving the pyramidion end dry. In this case a large allowance would have to be made in case the granite did not break evenly across between the wedge-channels. The great objection to this method is the risk of the obelisk breaking across owing to uneven strains set up by the wedges; it will be seen, in section [43], that the obelisk can only just support its own weight when in a horizontal position. If this method were employed, before the obelisk could be moved, it had to be raised off its bed to pass ropes round it. This could be effected by levering from both sides of the obelisk—using the outer trench wall as fulcra—and gradually rocking the obelisk higher by packing below it at each tilt. Assuming that only half a metre was undercut from both sides, it would require 30 12‐inch tree-trunks going down three feet into the trench (properly packed), and projecting 18 feet above the trench, being used vertically, with 70 men pulling on ropes attached to the top of each lever. The strain set up would be about 1000 pounds per square inch, which is well within the {24} powers of ordinary coniferous wood as cypress. (I assume that a man pulls 100 pounds.) The more undercutting performed, the less force would be required to rock the obelisk. Since the obelisk would have to be tilted from both sides, a good deal of rock would have to be removed from the south side of it before the levers could be used.

(22) (2) By completely undercutting the obelisk. In spite of the slowness of the work I am convinced that the obelisk was completely undercut, most likely by hand pounding, since the expenditure of copper chisels would be terrific, and the idea, in this kind of work, seems to be to economise as much as possible on copper. It would be packed by wooden blocks or stone as near the centre as is consistent with stability, and in as few places as possible. Ropes would then be passed round the obelisk, each going several times round and being brought forward from below to anchorages in front. It is here that the details of this method assume such great importance; it must be remembered that, if the obelisk weighs 1170 tons (allowing a margin for the roughness of the undercutting below), and lies on its side on a hard bed, then the horizontal pull by ropes necessary to turn it over on to a new face will be half the weight of the obelisk, i. e. 585 tons. This would need 13,000 men, if a man pulls 100 pounds. I do not see how such a number could possibly be put on to this work. Figure 6 shews the obelisk supported on its packing, the section here being at the centre of gravity, and the outer edge of the packing being 1 metre from the centre of the obelisk on each side. To pull it over by horizontal ropes would need 8000 men, which still seems more than is practicable. It is possible to reduce the number of men required to turn the obelisk over by means of levers working off the north wall of the north trench, which seems to have been deliberately left for that purpose (cf. section [5]).

Fig. 6.

(23) By using, say, 30 21‐foot levers with a mechanical advantage of six to one, as described in the last section, the obelisk can be made to turn slightly about N (see fig. [6]) so that the packing P can be removed and perhaps replaced by sand.