In a special reconnaissance, not only accurate but minute information will be required:  the character of the roads must be given in detail; the nature of the water-courses, their depth and velocity; the position and character of bridges, and fords;—­in fine, a full description of all obstacles to be encountered, and the means that can be made available for overcoming these obstacles.

A reconnoitring officer may usually derive much valuable information from the published maps and descriptions of the country to be examined; additional matters of detail may be obtained from woodsmen, hunters, and fishermen; and also from the innkeepers and local authorities of the district.  But the officer should always verify this information, so far as practical, by personal examination.  In making a reconnaissance in the vicinity of an enemy, he must be supported by a strong escort of mounted troops, and in all his operations the greatest precaution will be requisite to ensure success.

Some simple instrument, such as a pocket sextant, or compass, will be sufficient to enable the reconnoitring officer to measure, with considerable accuracy, the height of mountains, the width of streams, &c., and an ordinary scale and dividers will enable him to make a suitable military sketch.

Temporary Fortification.—­It has been stated in the preceding chapter that temporary fortifications are properly confined to the operations of a single campaign, and are used to strengthen positions which are to be occupied only for a short period; and that they are usually made of earth, thrown up by the troops in a single day.  Temporary fortifications, as a part of field-engineering, may therefore be regarded rather as an arm than an art.  The principles of their construction are derived, of course, from the theory of permanent fortification, but in applying these principles to practice in the field, much greater latitude is allowed than in the exact scientific arrangement of permanent works.

The purpose of field-works (or intrenchments, as they are commonly called) is to arrest, or at least to impede, the march of the attacking foe; to shelter the defensive troops from the missive weapons of the assailants, and to detain them in a position where they will be exposed to the fire of the defensive force.  The numerical and positive strength of the assailed may be much less than that of the assailant, and yet an equilibrium exist; the material obstacles compensating for the difference in numbers.  Intrenchments, though inert masses, must therefore be regarded as most valuable and important accessaries in the defence of a position.

Intrenchments consist either of lines of works made to cover extended positions, or of detached works designed simply to defend the ground they occupy.  The former generally present a front against the enemy in but one direction, while the latter are usually closed on all their sides.