FORTIFICATION. 15c 



glacis, also effect this object ; and fougasses, a kind of mine which we shall 

 soon describe, serve to destroy the assailing foe. 



g. Defilement. In the disposition of fortifications care must be taken 

 that they are not so placed as to be looked into from any adjacent heights. 

 By exact measurements, therefore, the command must be determined, and 

 the parapet made so high that it cannot be looked over, when the work is 

 said to be " defiled" or " defiladed ;" or else traverses are thrown up, or such 

 commanding heights as it is impossible to defile from are included within 

 the circuit of the works. The method of defilement is as follows : Let it be 

 the redoubt mnop (pi. 47, Jig. 14) which is to be defiled from the heights 

 ABB, the first step is to establish the plane of defilement, v t (figs. 15, 16), 

 so that it shall pass four feet six inches above the highest of the points 

 ABB, and through a point Z, four feet nine inches above the plain at the 

 foot of the glacis. In this plane v t, the crest of the ramparts n and p (fig. 

 15) must lie. These ramparts, however, would be very lofty, and yet not 

 cover the defenders upon the lines mp and p o (fig. 14), but, by having 

 recourse to a traverse m o, the height of the breastwork can be determined 

 by the plane of the defilement vp (fig. 16), which gives the angle p com- 

 mand over the ground Z. By the traverse, m, the defenders at p are 

 secured, while those at n are protected by the defilement itself. The dike 

 at D (fig. 14) is only to be considered when it is so high as to command jo, 

 in which case a new plane of defilement, and also the traverse which must 

 then protect n, are to be determined. Thus cases may arise where even 

 two traverses are requisite. 



h. Construction of Various Works in Fortification. As soon as the 

 disposition of the work is determined, it is traced upon the ground after the 

 plans, staked out, and then profiled. This last is done by setting up on all 

 the lines profiles of laths (pi. 47, fig. 18) and strips of board, for which the 

 profile given in fig. 17 is the original. To effect this, the distances v q, q o, 

 o h, h k, and k m, are staked off, and at each of these points strips of board, 

 longer or shorter as required, are driven into the ground ; on these strips the 

 proper heights are laid off, and then the slopes are given by cross laths, d c, 

 ch, ha, and fr, tacked to the strips. After the accuracy of the profile is 

 ascertained by measurement of the lengths ef and x y, the crest of the para- 

 pet is indicated by a stretched cord. During the construction, one third of 

 the force is detailed to cover the work ; of the remainder, three sevenths 

 are stationed in the ditch (figs. 19, 20), two of which sevenths, provided 

 with shovels, dig at K, while the other seventh, at L, loosen the earth with 

 picks. The shovellers, K, throw the earth into the berme, r s. Two sevenths 

 of the force are stationed with shovels upon the berme at M N, to send the 

 thrown-up earth backwards, and the remaining two sevenths stand at O and 

 P, upon the parapet. Half of these, O, have rammers ; the others, shovels 

 and spades ; and both spread the earth upon the parapet and form the slopes. 

 If the parapet is very high, and the ditches, therefore, very deep, they work 

 in two stages, by cutting a step along the counterscarp, as at x and y 

 (/^. 19). 



i. Block-Houses. As block-houses are very effective for the interior 



629 



