RECONNOISSANCE is an examination of a tract of country or of the seacoast; the latter previously to a disembarkation of troops, and the former preparatory to the march of an army in order either to meet that of the enemy or to take up quartets for the season.
The military reconnoissance of a country is a duty appertaining primarily to the officers on the staff of the quartermaster-general; though in many cases, such as sieges, &c., it is necessarily carried out by the engineers ; and if the enemy is in the neighbourhood, it is per formed under the protection of an armed force. It is considered as one of the most essential operations connected with the tactics of the field, and serves as the basis of every movemeut or combination which it may be proposed to make.
A general knowledge of a country which is or may become the seat of war—that is to say, a knowledge of the positions of its fortified places, the directions of its mountains, rivers, and great roads—may be obtained from maps or from geographical descriptions; but that which is necessary for the immediate purposes of a campaign can only be acquired by an actual survey of the ground in detail, and by inquiries made on the spot respecting the means which the country may afford for supplying the wants of the army.
The 3IarSchal Puyaegur (1690) appears to have been the first in modern warfare who regularly performed this duty by examining personally the tract through which the army was to march, or in which it was to encamp, and of deciding beforehand on the best routes and positions. He observes (' Art do la Guerre') that before his time it had been customary to trust for a knowledge of these points to the reports of the country-people, or of officers who might accidentally have been on the ground. Ho adds that disasters frequently occurred by the lines of march being improperly chosen, and that sometimes, after fatiguing marches, and after all the labour of encamping had been undergone, the troops had been compelled to abandon the positions on account of their unfitness. Since that time, armies being more numerous and more widely disseminated, consequently requiring more vigilance in the communication of one part with another, and a greater extent of country for their support, the reconnoissancee have been made on a greater scale, and in the military establishments of every nation officers are now particularly instructed iu all the details of that branch of service.
Those who are charged with this duty should be habituated to the performance of topographical suiveye : in the first place by the most accurate methods and with the best instruments ; and secondly, by such methods as admit of being practised rapidly on foot or on horse back. In these cases, an azimuth, compass held in the hand must be used for observing the angles, and the distances must be obtained by pacing, or be merely estimated by the eye. A micrometer scale, divided to 200ths of an inch, fixed in the eye-piece of a telescope, will give distances with comparatively great accuracy and great ease, as it is only necessary to see how many divisions an object subtends, and on arriving at it to measure it, the focal length of telescope having been previously determined by experiment. A facility in representing on
the plan the inequalities of the ground is also highly necessary.
In making the reconnoissances previously to the march of en army, the whole of the ground between the actual position of the latter and that which is intended to be occupied should be surveyed if the enemy Is near and there is danger of his attacking the columns by surprise ; otherwise it may be sufficient to survey the ground within a few hundred paces on each side of the roads by which the columns are to march. A complete plan of the tract of country in which the recon noissance is made may therefore be required ; or it may suffice to represent on paper the line or lines of march. In either case the officer may be provided with a general map, or an itinerary of the intended route, as an outline for his guidance ; and his survey, when completed, should be accompanied by a report or memoir, stating in detail what cannot be conveniently represented on the plan. In this report should be expressed, with all necessary references to the plan of the ground, the distances, by the different routes, between the two positions, and the places where troops may halt for repose or to form in order of battle, distinguishing particularly the plains where cavalry may act, and the heights on which artillery may be placed. The nature of the roads should be described, with indications denoting that they are passable for artillery, for cavalry, or merely for infantry ; and if defective, estimates should be made of the materials and time requisite for repairing them. It is particularly necessary to state whether the ascents and descents are gentle or abrupt ; aud, when the road is on the side of a bill, whether it is sufficiently level to allow artillery or carriages to pass safely. It should also be noticed whether or not, at places where roads run through towns, they are reduced to narrow and winding streets. The breadths and velocities of rivers, streams, and canals which cross the lines of march should be ascer tained, and a statement made whether the beds are rocky, gravelly, or muddy ; also whether the banks are high or low. Mention should also be made of the means which exist for passing them ; of the places where they are fordable, where there are ferries or bridges, or where boats may be procured : descriptions should also be given of the bridges or boats, and the manner of working the latter. The situations and extent of marshes should also be shown, and it should be stated whether they are passable or can be made so. In contemplating rivers and marshes as means of retarding an advance of the enemy, it should be ascertained and reported whether, by being dry in summer or frozen in winter, they may not at times cease to be obstacles. It should also be stated how, on a the roads may be blocked up, the fords rendered impassable, or the bridges destroyed.