If what Vegetius relates of the helepolis be true,--and, indeed, with a small variation, Vitruvius seems to confirm it,--it might possibly be another motive that contributed not a little to dispose Demetrius to a peace. He was preparing to advance his helepolis against the city, when a Rhodian engineer contrived an expedient to render it utterly useless: he opened a mine under the walls of the city, and continued it to the way over which the tower was to pass toward the walls the next day. The besiegers, not expecting a stratagem of that nature, moved the tower on to the place undermined, which being incapable of supporting so enormous a load, sunk in under the machine, which buried itself so deep in the earth, that it was impossible to draw it out again. This was one inconvenience to which all these formidable machines were obnoxious; and the two authors cited declare that the accident determined Demetrius to raise the siege; and it is at least very probable that it contributed not a little to his taking that resolution. The Rhodians, on their part, were as desirous of an accommodation as himself, provided it could be effected on reasonable terms. Ptolemy, in promising them fresh succours much more considerable than the former, had earnestly exhorted them not to lose a favourable opportunity, if it should offer itself. Besides which, they were sensible of the extreme necessity of putting an end to a siege which must prove fatal at last. This consideration induced them to listen with pleasure to the proposals made them, and the treaty was concluded soon after, upon the following terms: That the republic of Rhodes and all its citizens should retain the enjoyment of their rights, privileges, and liberty, without being subjected to any power whatever. The alliance they had had with Antigonus was to be confirmed and renewed, with an obligation to take up arms for him in any war in which he should be engaged, provided it was not against Ptolemy. The city was also to deliver a hundred hostages, to be chosen by Demetrius, for the effectual performance of the stipulated articles. When these hostages were given, the army decamped from before Rhodes, after having besieged it a year.