This comparatively high cost of the bridge has had certain effects on the history and in the appearance of our roads which are worth noting. In the first place, the bridge tends to be a “gut.” When the throwing of a bridge was equivalent in expense to several miles of the existing road it was a great saving to make it narrow: only one vehicle to pass at a time, with side refuges at the piles when the passage of two vehicles in opposing directions was unavoidable. Again, bridges tended, especially in times of low economic development, to introduce a sudden high gradient. The elliptical arch was, if not unknown, at any rate very rare before the Renaissance, and where the plain semi-circular arch alone was used a flat bridge involved, if the crossing were of any width, a great number of piles, and therefore an added expense. The difficulty was met in the majority of cases by lessening the number of piles, especially towards the centre, where there was a greater depth, consequently increasing the span there, and consequently, in a semi-circular arch, increasing its height correspondingly. The result was that the bridge introduced a sudden hillock into the Road, and that feature you find all over Western Europe up to quite modern times, with many survivals remaining, especially in Spain. In some of the very early bridges in the poorer districts, or on the less used roads, the exaggeration is fantastic. I know of one over the Gallego, near Huesca, where the pitch is so steep that it baulks a car. There were particular structures--that of London is an example in point--where the disadvantage of a gradient was avoided at great expense because a mass of traffic and merchandise made it worth while. London Bridge was carried on a great number of arches precisely in order to avoid this element of gradient. A side-effect of this was the blocking of the stream and great difficulty for boats in “shooting” the arches on a tide; but this drawback to river traffic was thought worth while as the price of a level road. Another reason which often led to the expensive flat stone bridge was its replacing an old wooden pile bridge. The wooden pile bridge had no cause for creating a gradient. On the whole it was cheaper to keep it exactly level, and as low as possible consistent with the rise of the water. Where such a structure had preceded a stone bridge the habit of a level road was continued, even at the expense of many piles and arches.