Military art has devised many expedients for bridging streams, and use is made of any facilities that may be at hand for constructing the means of passage; but the only organized bridge trains which move with the army are those which carry the pontoons. Of these there are various kinds, made of wood, of corrugated iron, and of india rubber stretched over frames. But the wooden pontoon boats are most in use. They can be placed in a river and the flooring laid upon them with great rapidity. Several very fine bridges have been thus constructed--among them may be mentioned the one at the mouth of the Chickahominy, across which General McClellan's army marched in retreating from Harrison's Landing. It was about a mile long, and was constructed in a few hours. To cross a river under the fire of an enemy is one of the most difficult operations in warfare. Yet it has been frequently accomplished by our armies. The crossing of the Rappahannock by General Burnside's army, previous to the great battle of Fredericksburg, in December, 1862, is one of the most remarkable instances of the kind during the war. The rebel rifle pits lined the southern bank, and the fire from them prevented our engineers from approaching--the river being only about seventy-five yards wide. For a long time our artillery failed to drive the rebels away. About noon of the day on which the crossing was made, General Burnside ordered a concentration of fire on Fredericksburg, in the houses of which place the rebels had concealed their forces. A hundred guns, hurling shot and shell into every building and street of the city, soon riddled it; but the obstinate foes hid themselves in the cellars till the storm was over, and then emerged defiantly. They were only dislodged by sending over a battalion in boats to attack them in flank, when they retreated, and the bridges were laid. It is impossible to explain in articles of this character the mysteries of intrenchment and fortification, so that they will be comprehensible. A few notes, however, on some of the principal terms constantly employed, may be found useful and interesting. _Rifle pits_--as the term is now generally used--are small embankments, made by throwing up dirt from an excavation _inside_. They can be erected quickly, for it will be seen that those behind them have the advantage, not only of the height of the embankment, but also of the depth of the ditch. Thus an excavation of two feet would give a protection of _four_ feet. This is the ordinary rifle pit, but when time permits it receives many improvements. _Breastworks_ are any erections of logs, dirt, etc., raised breast high, to shelter the men behind them.