placed in the middle of the road and each side made to slope slightly towards it. BRIDGES. The earliest efforts of the civil engineer were in all probability directed to the construction of roads, but it would be evident that unless they could be made to cross rivers and water-courses, they could continue but a short distance in any required direction. Bridges of some sort must therefore be constructed; small brooks were doubtless bridged over with timber laid across them, and “fords” or shallow parts of rivers taken advantage of when the stream was too wide to be spanned in this way. That these fords were once very numerous in England, is shown by the constant recurrence of such names as Stratford, Brentford, &c., and indeed fords are now in use in many places where the stream is shallow, and the traffic too little to pay for constructing a bridge. The next improvement in all probability consisted in placing stones in the fords to enable passengers to step from one to the next, and cross the stream dry-footed, which were called “stepping stones;” but the earliest bridges properly so called, were probably stones piled upon each other and united by beams of timber. Illustration: VIADUCT. The Chinese, Romans, and several other nations were acquainted with the use and proper mode of constructing bridges, many centuries ago, when no such works had been attempted in England. The materials chiefly now in use for bridges are stone, brick, iron, and wood, and by far the greatest number, till within the last half century, were constructed of stone, but iron seems to be rapidly taking its place, especially since the mode of constructing suspension bridges has been more perfected. There are several ways of forming the foundation for the piers, or those parts on which the bridge rests. Old London bridge and many others of considerable size were based upon “piles,” these piles consist of great pieces of timber pointed at one end and driven deep down into the bed of the river close together, so as to form a solid mass on the top of which the stones for the piers were laid; these piles have been found scarcely altered after several centuries of immersion in the mud, but at the parts exposed to the action of the water they soon become decayed and eaten away by its inhabitants. For many bridges, when there is good foundation, the stonework is laid at once on the bed of the river, but in order that this may be got at, a double ring of piles is driven close together around the part where the pier is to be built; these piles are of a flat form, and the two rows inclose a space of about a foot between them, which space is filled with clay, well rammed in, and the water within the barrier pumped out, this forms what is called a “coffer-dam,” and is an expensive proceeding. The bottom of the river thus exposed can be built upon in the usual way. Another way of forming a foundation for piers consists in lowering large wooden