To prevent a surprise, detachments of light troops should be always thrown out in front, on the flanks, and in rear of the column, denominated from their position, _Advanced-Guard, Flankers,_ and _Rear-Guard._ These scan the country which is to be passed over by the column, watch the enemy's motions, and give notice of his approach in time to allow the main force to choose a suitable field of battle, and to pass from the order of march to that of combat. The strength and composition of these detachments depend upon the nature of the ground, and the character and position of the enemy. In case of an attack they retire slowly, and on joining the main body, take their assigned position in the line of battle. In an open country the order of march presents but little difficulty; but in a broken country, and especially in the vicinity of the enemy, a march cannot be conducted with too many precautions. Before engaging in a _defile_ it should be thoroughly examined, and sufficient detachments sent out to cover the main body from attack while effecting the passage. A neglect of these precautions has sometimes led to the most terrible disasters. In military operations very much depends upon the rapidity of marches. The Roman infantry, in Scipio's campaigns in Africa, frequently marched a distance of twenty miles in five hours, each soldier carrying from fifty to eighty pounds of baggage. Septimius Severus, Gibbon states, marched from Vienna to Rome, a distance of eight hundred miles, in forty days. Caesar marched from Rome to the Sierra-Morena, in Spain, a distance of four hundred and fifty leagues, in twenty-three days! Napoleon excelled all modern generals in the celerity of his movements. Others have made for a single day as extraordinary marches as the French, but for general activity during a campaign they have no rivals in modern history. A few examples of the rapidity of their movements may not be without interest. In 1797 a part of Napoleon's army left Verona after having fought the battle of St. Michaels, on the 13th of January, then marched all night upon Rivoli, fought in the mountains on the 14th, returned to Mantua on the 15th, and defeated the army of Provera on the morning of the 16th,--thus, in less than four days, having marched near fifty leagues, fought three battles, and captured more than twenty thousand prisoners! Well might he write to the Directory that his soldiers had surpassed the much vaunted rapidity of Caesar's legions. In the campaign of 1800, Macdonald, wishing to prevent the escape of Loudon, in a single day marched forty miles, crossing rivers, and climbing mountains and glaciers. In 1805 the grand French army broke up their camp at Boulogne, in the early part of September, and in two weeks reached their allotted posts on the Rhine, averaging daily from twenty-five to thirty miles.