148 MILITARY SCIENCES. 



taigne, and Montalembert, have brought this branch of military science to 

 a very high degree of perfection 



1. Field Fortification, 



The object of every fortification is so to surround a spot with obstacles, 

 that a division of troops occupying the same may defend themselves with 

 advantage against superior numbers. If only the passing movements of an 

 army are to be thus supported, the fortifications are but simple. A funda- 

 mental principle is, that every point of the work shall be swept by two fires, 

 a direct and a cross or flanking fire, taking the enemy on the side ; the 

 distance, therefore, from one flanking point to another must never exceed 

 good musket range, that is, from 300 to 480 feet. 



Field works are divided into three classes : open works, closed works, 

 and fortified lines. 



a. Open Works. All works not entirely inclosed by their parapet are 

 said to be open. They are, according to their form, 1, simple redans or 

 tenailles, which consist of the straight lines of fire (faces) meeting each other 

 under a greater or less angle, sometimes broken and furnished with flanks. 

 If it is desired to protect the salient by a cross-fire, the flanks are broken 

 to the front {pi. 47, fig. 5). The redan hah has the flanks cd, which 

 defend the dead-angle in front of the salient a by a cross-fire. The flanks 

 must never be more than half musket-shot distance from the salient angle, 

 and must stand perpendicularly to the face on which they belong. If the 

 ground is too extended for a simple redan, it is then doubled {pi. 41, fig. 6, 

 g ab a g), and here also the flanks b d c can be broken to the front. Such 

 double redans are called swallow-tails, and double swallow-tails when there 

 are three salient and two re-entering angles {fig. 7), in which case the 

 flanks are still broken forward. The salient angles must not be less than 60°, 

 the re-entering not less than 90°. If the extent of the space to be defended 

 or other local circumstances render it requisite, a system of tenailles is 

 employed {fig. 9) ; the side of the polygon must not, however, exceed 180 

 feet. When a longer polygon side than this is to be defended, a a {fig. 10), 

 it is broken repeatedly and a bastion constructed upon it. For this purpose 

 the triangle a a a is, constructed, from the two new and the old polygon 

 sides, and in the centre c of the new polygon side the perpendicular c 6 is 

 erected, which for the square is \, for the pentagon |, and for the hexagon, 

 &c., ^ of the side in length. Through all the points a and h, undetermined 

 lines, a x, are drawn, and a d made equal io ^ aa, by which the faces of the 

 bastion are given. From d perpendiculars are let fall upon a x, thus obtain- 

 ing the flanks of the bastion, and if now the extremities of the opposite 

 flanks are united by straight lines, the curtains, we have the complete trace 

 of the bastioned front. 



h. Inclosed Works. If the point to be defended is exposed to attack 



on all sides, the defences must then surround it, thus forming an inclosed 



work. The dimensions of such works, and consequently the length of their 



polygon sides, depend upon the strength of the assigned garrison, and 



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