Two outstanding examples of this in Europe are London Bridge, perhaps prehistoric, certainly not much less than two thousand years old, and the bridge at Cologne, to which one might add the bridge at Rouen and the bridges of the Island of Paris, which we know to be more than two thousand years old. But it must be remembered that the bridging of a river, even in primitive times, was the next easiest thing to a ferry, and in some circumstances easier even than a ferry. A bridge need not be built of piles. It may be built of boats, and in principle, even over a broad stream, once you could build a boat bridge at all you could build it of almost indefinite length. What would militate against the effort to make a pile bridge were depth and rapidity of stream, but even these, unless the rapidity were very great indeed, did not prevent the throwing of a bridge of boats. The bridge as an element in the Road plays a very large part which needs some detailed examination: it develops a whole series of results. The object of a bridge is to give _continuity_ and _security_ to travel across an obstacle of depth: usually an obstacle of running water, sometimes a dry ravine. _It is but rarely that a bridge is essential to the mere trajectory of a road._ In much the greater number of cases its function can be supplied, though far less perfectly, by a ferry, or a ford, or a graded way down into and up from a depression. What the bridge does is to permit of continued traffic, especially continued wheeled traffic, across such obstacles without delay and without trans-shipment, and at the same time to add, up to a maximum of weight, to security; for it is obviously an instrument more secure than the ferry or the ford, especially for heavy weights. But the bridge has always represented a special economic effort, greater yard for yard than that of the average of the road of which it was a part; and that is why you almost always find it the mark of civilization. A primitive culture can exist for centuries without bridges. The proportion of bridge-building effort to road-building effort varies very much with the physical science of various times. It is less to-day, and was less in Roman times, than in primitive times and in the Middle Ages, because we, like the Roman engineers, expend a far greater economic effort upon the average of the Road, so that the comparative cost of the bridge is less. In primitive times the bridge was something of a feat, its construction as measured in effort was equivalent to many miles of road, its builder a public benefactor, and its building an event of note. This is so true that in some languages which have come down but little changed from primitive times the word for “bridge” is found to be a foreign word, as though the institution were not sufficiently common before the advent of some civilized conqueror to have acquired a special name; and in all primitive societies the bridge is rare.