Equally well known, no doubt, are the gantries built over the footway while a large building is in course of construction. Generally of huge square baulks of timber, they are intended to carry very heavy loads of materials and to save the public passing beneath from any possibility of damage through heavy objects falling from above. Those gantries furnish us with an example of another sort of construction in wood which can be and is often used in bridging. When the Germans retired in Northern France they blew up all bridges behind them, and before the Allies could use those bridges they had to repair them. If only for foot-traffic, a contrivance of poles, lashed together after the manner of the builder's scaffold, is ample in most of such cases and by its means a strong and safe bridge can be made upon what is left of the old bridge in the course of a few hours. For light vehicles a similar structure but made stronger by more lashings and of poles closer together will suffice, but for heavy traffic, with guns and possibly railway trains, recourse has to be had to the heavy timberwork exemplified by the builder's gantry. This takes longer to make, since the timbers are big, heavy and not easy to move about: they are, moreover, not simply laid beside or across each other and tied, but are cut the right lengths, and one is notched where the end of another fits into or against it. The baulks are connected by bolts and nuts for which holes have to be drilled or by rods of iron with a sharply pointed prong on each end stretching across from one baulk to another, one prong being driven into each. With the long-thought-out military operations of modern warfare it is just possible that steelwork for repairing certain particular bridges might be prepared in advance and simply launched across when the time arrives, but that is manifestly impossible except in certain cases and under particularly favourable conditions, such as railway facilities for bringing up the new bridge close to the site where it is to go. Nearly every military bridge therefore has to be more or less improvised on the spot. In a highly developed country scaffold poles or baulks may be found or brought up by road or rail, in less civilised lands their equivalents may be cut and prepared from neighbouring forests, but all armies have, as a recognised part of their organisation, certain engineering "field companies," and "bridging trains," which carry with them large quantities of material carefully schemed out long in advance, so shaped and so prepared that it can be fashioned into almost anything, much as the strips of a boy's "meccano" can be adapted to form a great variety of objects.