Bridges
The bridges are like long arms thrust out to grasp some other point or shore.
One of man’s earliest problems was how to cross water. He could, of course, swim or wade or even ferry across in a roughly made bode. He might use stepping stones if the water was not deep. He might even make a ford by laying flat stones close together in some shallow level part of the river bed.
At last man hit upon the plan of building a bridge to cross a river or fake. This might be done simply as they do in Tibet by felling a tall straight tree close to the stream’s edge and allowing it fall across to the opposite bank. Then .wooden bridges were built upon supports which rested on the bed of the stream. The light traffic of those days could pass with ease and comfort over those bridges.
The greatest bridge builders of olden times were the Romans for their vast empire made it necessary to move armies rapidly from place to place. At first their bridges even those built in the city of Rome and across Thames at London were made of wood.
Since wood was liable to rot or to be washed away in time of flood, they began to build stone bridges. Such bridges were built upon arches for the Romans were the first people to understand fully the great strength of a stone arch. Some of these Roman bridges still stand proudly in various parts of Europe and North Africa, though they have of course been repaired from time to time.
In later times handsome stone bridges were bull over most of the great rivers of Europe. Over some of them stood shops and houses as well as a roadway while the river flowed beneath. The most famous of such bridges was the London Bridge which was built about the year 1200. It remained for six centuries holding up to load of houses, bookshops and tavern and but the bridge collapsed more than once.
In the nineteenth century the great increase in trade and especially the coming of the railways, made bridge building more important than ever. Going to new methods and discoveries, it become possible to use iron later steel as a ridge building material instead of stones. Longer and stronger bridges could be built now.
Engineers had to make many experiments and some-times there were curious results. Sometimes there were failures and disasters as in France, a bridge collapsed while regiment of soldiers was marching across and many of the soldiers were drowned.
But engineers like all wise people are glad to learn from failures and even from disasters. As a result greater and stronger and even more wonderful bridges are being built today.
