method of bridging rivers.

how alexander constructed his bridge over the river indus, is explained neither by aristobulus nor ptolemy, authors whom i usually follow; nor am i able to form a decided opinion whether the passage was bridged with boats, as the hellespont was by xerxes and the bosporus and the ister were by darius,663 or whether he made a continuous bridge over the river. to me it seems probable that the bridge was made of boats; for the depth of the water would not have admitted of the construction of a regular bridge, nor could so enormous a work have been completed in so short a time.664 if the passage was bridged with boats, i cannot decide whether the vessels being fastened together with ropes and moored in a row were sufficient to form the bridge, as herodotus the halicarnassian says the hellespont was bridged, or whether the work was effected in the way in which the bridge upon the ister and that upon the celtic rhine665 are made by the romans, and in the way in which they bridged the euphrates and tigres, as often as necessity 278compelled them. however, as i know myself, the romans find the quickest way of making a bridge to be with vessels; and this method i shall on the present occasion explain, because it is worth describing. at a pre-concerted signal they let the vessels loose down the stream, not with their prows forward, but as if backing water.666 as might naturally be expected, the stream carries them down, but a skiff furnished with oars holds them back, until it settles them in the place assigned to them. then pyramidal wicker-baskets made of willow, full of unhewn stones, are let down into the water from the prow of each vessel, in order to hold it up against the force of the stream. as soon as any one of these vessels has been held fast, another is in the same way moored with its prow against the stream, distant from the first as far as is consistent with their supporting what is put upon them. on both of them are placed pieces of timber with sharp ends projecting out, on which cross-planks are placed to bind them together; and so proceeds the work through all the vessels which are required to bridge the river. at each end of this bridge firmly fixed gangways are thrown forward,667 so that the approach may be safer for the horses and beasts of burden, and at the same time to serve as a bond to the bridge. in a short time the whole is finished with a great noise and bustle; but yet discipline is not relaxed while the work is going on. in each vessel the exhortations of the overseers to the men, or their censures of sluggishness, neither prevent the orders being heard nor impede the rapidity of the work.