A GREAT many hundred years ago Xerxes, a Persian king, collected the largest number of men ever 

known to have assembled at any one time. 

He was going over into Greece to conquer the people of that country. But he failed to accomplish his object, for the Grecians were a very brave and determined people, while the Persians were accustomed to ease and luxury. A great many things happened in connection with this expedition, which are very interesting to read about. Among the rest is the way in which Xerxes carried his immense army over the Hellespont, a wide strait of stormy water, separating Asia from Europe. His army was so vast that it would have taken weeks and weeks to carry them over in boats, and yet the water was too deep, and the strait too wide to think of building a bridge. 

So Xerxes ordered his men to place two lines of boats, lying side by side, across the strait, and then to build a floor, using the boats for piers, or supports. When this was done, the baggage and horses were taken across on one bridge, while the men crossed on the other. Although both men and beasts were hurried across by being driven with the lash, yet so mighty was the host that they were seven days and seven nights in crossing. 

Since the days of Xerxes, many other generals have made use of boats to build bridges upon, and now scarcely any army thinks of starting out on a campaign without carrying the timber necessary to build a "pontoon bridge," as they are called. With such a bridge, they can cross rivers at any point they choose, and can carry off their bridge with them, and not leave it for their enemies to use. The picture will give some idea of how these bridges are built, and how they look. 

C. A. G