A third effect of the bridge upon the Road, also due to its comparative expense, was the convergence of roads towards bridges, established or even only planned. You will perpetually find up and down Europe the approaches to a town from two or more directions merged into a common road just at the entry to a bridge, in order to save the expense of two crossings, though at an extra expense of space and time; thus, Abbeville, Caen (a very striking example, with _three_ converging roads on each side of the bridge), London--the chief example in Europe--Saragossa, with the two main roads from south and west converging on its bridge--all “gather” roads after this fashion. But the effects of the bridge upon the mere trajectory of a road, upon its surface and contour, were far less than were its political and military effects. Though land armies were always tied to roads more or less, it was possible to leave the road for short distances under stress or for the sake of strategy. Cavalry continually did so for great stretches, and infantry could do so occasionally. But a bridge acted like a magnet. The defence of a bridge was the defence of a point which an army in force was always compelled to use, and the term “bridge head”--that is, the holding of the space on the _further_ side of the bridge, thus commanding the passage--is an example of its permanent military function. A bridge was, for the same reason, a natural place of toll. Merchandise had to use it, and the same requirement of continual repair which often entailed a permanent post at a bridge gave the opportunity for using that post for the raising of taxation. All through the end of the Roman Empire and the Dark and Middle Ages this function of the bridge is most prominent. iv But most important of all the effects of the bridge is its creation of a _nodal point_, that is, a knot or crossing of ways. The bridge effects this in two fashions: firstly by that tendency to a convergence of roads upon the bridge which I have just noted, and secondly, and much more important, by the transverse of the bridge and the river. A river is also a high road if it is in any way navigable. Therefore, wherever a land road crosses a river and establishes a bridge you get a crossways of communications. At such a point, where many avenues of approach meet, and whence opportunities of travel to different places radiate, you have what is called in political geography a Nodal Point. [Illustration: ICKNIELD STREET in the Oxfordshire Chilterns]