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. The following figures have been employed for the plan of simple intrenchments, viz.: the polygon, redan, lunette, mitre, star-fort, and bastion. _Square_ or _polygonal redoubts_ are the most common forms given to field-works, on account of the ease of their construction. But they have many defects. There is a sector without fire in front of each salient, and the ditches are without protection. The latter objection also holds good against all circular works. The _redan_ (Fig. 45) is frequently used to cover a point in rear, as a bridge, a ford, or a defile. When used alone, its gorge should be closed by palisades. Its ditches are unprotected. The _lunette_ (Fig. 46) has nearly the same defects as the redan. The _mitre_, or _priest-cap,_ (Fig. 47,) may be employed with advantage when a cross-fire is required on the capital of the work. The _star-fort_ has all the defects, without the merit of simplicity, which belong to the polygonal redoubt.