Here, moreover, Trajan saw the asphalt out of which the walls of Babylon had been built. When mixed with baked bricks or smooth stones this material affords so great strength as to render them stronger than rock or any kind of iron. He also looked at the opening from which issues a deadly vapor that destroys any creature living upon the earth and any winged thing that so much as inhales a breath of it. If it extended far above ground or had several vents, the place would not be inhabitable; but, as it is, this gas circles round within itself and remains stationary. Hence creatures that fly high enough above it and such as remain to one side are safe. I saw another opening like it at Hierapolis in Asia, and tested it by means of birds; I bent over it myself and myself gazed down upon the vapor. It is enclosed in a sort of a cistern and a theatre had been built over it. It destroys all living things save human beings that have been emasculated. The reason for that I can not comprehend. I relate what I have seen as I have seen it and what I have heard as I have heard it. [Sidenote: A.D. 116 (a.u. 869)] Trajan had planned to conduct the Euphrates through a channel into the Tigris, in order that boats might be floated down by this route, affording him an opportunity to make a bridge. But on learning that it had a much higher elevation than the Tigris, he did not do it, fearing that the water might rush pell-mell down hill and render the Euphrates unnavigable. So he conveyed the boats across by means of hauling engines at the point where the space between the rivers is the least--the whole stream of the Euphrates empties into a swamp and from there somehow joins the Tigris--then crossed the Tigris and entered Ctesiphon. Having taken possession of this town he was saluted as imperator and established his right to the title of Parthicus. Various honors were voted him by the senate, among others the privilege of celebrating as many triumphs as he might desire. After his capture of Ctesiphon he felt a wish to sail down into the Red Sea. This is a part of the ocean and has been so named [Footnote: [Greek: erythra] from Erythras, who was said to have been drowned in it (as if in English we should invent a King Redd).] from some person formerly ruler there. Mesene, the island in the Tigris of which Athambelus was king, he acquired without difficulty. [And it remained loyal to Trajan, although ordered to pay tribute.] But through a storm, and the violence of the Tigris, and the backward flow from the ocean, he fell into danger. The inhabitants of the so-called palisade of Spasinus [they were subject to the dominion of Athambelus] received him kindly.