GIFT OF ^rv. °^.Q,.ej>X\kstil Digitized by the Internet Archive in 2008 with funding from IVIicrosoft Corporation http://www.archive.org/details/fortificationfieldOOwheerich THE ELEMENTS Field Fortifications FOR THE USE OF THE CADETS OF THE UNITED STATES MILITARY ACADEMY, AT WEST POINT, N. Y. BY J. B. WHEELER, PROFESSOR OF CIVIL AND MILITARY ENGINEERING IN THE U. S. MILITARY ACADEMY AT WEST POINT, N. Y., AND BREVET COLONEL, ,UN^T;f;D STATES ARMY. New York : D. VAN NOSTRAND COMPANY, 83 MURRAY ST. AND 27 WARRKN ST. 1893. / C?rKlGHT. \S&). U. VAN NO^TRAMD. ? / " T%ere are Jtve things from which the soldier muH never be acparatad : his f/uii. his ammunition, his knapsack, his rations for four days and an in- trenching tod" Napoleon. 251006 PREFACE This text-book is prepared for the use of the cadets of the United States Military Academy while pursuing their course of studies in the subject of *' military engineering." The endeavor has been made to state concisely and plainly the principles upon which the " art of fortification " is based, and to give all information likely to be of practical use to a young officer while serving in the field. All unnecessary details have been avoided, leaving explanations and illustrations of that kind to be introduced into the class-room. The elementary form of the work and the method of treatment of the subject are based upon the as- sumption that the readers of the book are beginners and know nothing of the principles of fortification. West Point, N. Y.. February, 1880. CONTENTS CHAPTER I. General Principles and Definitions. Article. Paqi 1. Definition of the term, position 7 2. Strong positions 8 o. Fortifications, their object 8 4. Classes of fortifications 9 5. Field fortifications 10 6. Kinds of field fortifications 10 7. General principles to be observed in the construction of fortifications 11 8. General methods followed to fulfil these conditions ... 12 9. Active and passive defence of a fortification 12 10. Definition of the term, ditch 13 11. The site of a work 14 12. The meaning of the term, intrenched position 14 CHAPTER II. Elements of the Profile of a Fortification. 13. The parapet 15 14. The terrepleiu , 15 15. The profile 15 16. The berm 18 17. Theditch 18 18. The foot of a slope, the crest, etc 18 19. The principal lines of the profile 19 n CONTENTS, CHAPTER III. Dimensions and Inclinations given to the Lines and Slopes of a Parapet. A.RTIOLB. PAGE 20. Height that a parapet should have 21 21. Definition of the terms, command and relief 22 22. Thickness that a parapet should have 22 23. Width and height of banquette 24 24. Inclination given to the interior slope 23 25. Inclination given to the superior slope 27 26. Inclination of the exterior slope 28 27. Width and useii of a berm 28 28. The uses, and the dimensions of a ditch . . 29 29. Slope of scarp and counterscarp 30 30. Method of determining tlie dimensions to give the ditch 30 31. Method used in practice 32 82. The normal profile of a field fortification 33 CHAPTER IV. The Trace of a Field Fortification. 33. Definition of the terms, trace and sub-crest 34 34. The kinds of fire to which a fortification, or a body of troops may be exposed 34 35. The salient and re-entering angles of a trace 36 36. The general principles to be observed in the selection of a trace 36 87. The reasons for observing these principles . . 37 CHAPTER V. Field Works. 88. Classification of field fortifications into field works and lines 39 39. Classes of field works » 39 CONTENTS. , Vll 4RTICLB. PAOl 40. Open, and half-closed field works 41 41. Lunettes 42 42. Bastioned front 42 43. Closed field works 43 44. Redoubts 43 45. Star redoubts 44 46. Bastioned forts 45 47. Nomenclature of the bastioned fort 46 48. Sectors without fire, and dead spaces 47 49. Flanked dispositions 48 50. Relations existing between the different parts of a bas- tioned front 4t> 51. The least and the greatest exterior sides that can be used in bastioned fronts 51 52. Defects to be found in a bastioned fort 52 63. Defects of star forts 53 CHAPTER VI. Lines. 54. Different classes of lines 54 55. What is meant by a continued line 54 56. What is meant by a line with intervals 54 Continued Lines. 57. Different kinds of continued lines 55 58. Tenaille line 55 59. Redan line 57 60. Indented lines 57 61. Bastioned lines 58 62. Advantages and disadvantages of continued lines 58 Lines with Intervals. 68. The positions occupied by the field works forming a line with intervals 59 64. Third line 61 65. The advantages and defects of lines with intervals com- pared with other kinds of lines 62 Vlll CONTENTS, CHAPTER VII. The size of a Field Work, the number of its Garrison, and the Selection of its Trace. AUTIOLE. PAGE 66. The size to give to a field work 64 67. Tlie number of the garrison to defend a field work. ... 65 68. The amount of space to be enclosed 66 69. The relation between the space, and the length of inte- rior crest of a square redoubt when the number of the garrison is known 67 70. The things to be observed in the selection of trace 09 CHAPTER VIII. Construction of Field Works. 71. The operation known as " tracing " 73 72' The operation known as "profiling" 73 73. The method employed to construct the parapet 76 74. The time which is required to build a field work 78 CHAPTER IX. Revetments. 75. What is meant by the term, revetment 81 76. Kinds of materials used for revetments 81 77. Fascines, and their construction 82 78. Fascine revetments 83 79. Gabions, and their construction , , . 85 80. Hurdle revetments 87 81. Revetments of timber 87 82. Revetments made of planks 89 83. Revetments made with casks, barrels, etc 90 84. Sand-bags, and tlitVir use in revetments 90 85. Sod revetments, and their construction 92 86. Pisa revetment, and its construction 93 87. Revetments of other kinds. 94 CONTENTS. 11 CHAPTER X. Defilade. UmOI,K. PAOl 88. What is meant by defilade, and how a field work may be defiladed 96 89 Plane of defilade, and its use 97 90. The amount of area to be defiladed 98 91. The method used to obtain the position of the plane of defilade 99 93. Front and reverse defilade 102 94. Methods used to defilade a work from reverse fires. ... 104 95. The method used to determine the position of the plane of reverse defilade 1 06 96. The method of defilade by means of traverses 108 97. Defilade of a line. . : 110 CHAPTER XI. The Interior Arrangements made in a Field Work. 98. Classes of interior arrangements 114 Arrangement of the Parapet. 99. Arrangement of the parapet for defence 115 100. Barbette fire of artillery 115 101. Method of constructing the trace of a barbette 116 102. Pan-coupe, when used 118 103. Embrasure fire of artillery 120 104. Method of constructing the trace of an embrasure 121 105. Direct and oblique embrasures 124 106. Comparative advantages and defects pertaining to bar- bette and embrasure fires. 125 107. Bonnettes, and their uses. 125 108. Loop-holes, and their object . 126 109. Traverses, their kind and their construction 128 110. Construction of sr-pro<)f traverses 130 111. Platforms for artillery in field works 132 Z CONTENTS. Arrangements for Sheltering the Troops, etc. from the effects of the Enemy's Fire. ARTICLE, P Ix— ^^i - \ S' cot. o V . • . (6) From these equations (5 and 6) y can be assumed, and X deduced ; or x assumed, and y deduced. Making = 45°, these last equations reduce to X 7 -S^ , 6 / > j^y-f -. and j, = y (^« " y «'^ - |^ It should be remembered in assuming values for 7L and y, that x must not be less than twelve feet, and y not less than six, nor greater than twelve feet. 31. In practice, it will be sufficiently accurate to calculate the area of the assumed profile, assume a depth for the ditch, and, without making an allowance DIMENSIONS OF AN ORDINARY PARAPET, 33 for the increase of volume of tiie earth in the embank- ment, divide the area of the profile by the assumed depth of the ditch. The result will give the width of the ditch at the top. 32. Normal profile.— The profile (Fig. 2) with ine dimensions and inclinations just mentioned is some- times called the normal profile of field fortifications. It is the profile which would be constructed for a work located upon a level site, and when there is time to build it. It is evident that great variations must occur, influenced largely by the kind of earth and the sur- rounding circumstances at the time of construction. Slopes which are practicable in one kind of earth will not retain their shapes in other kinds. Parapets placed on sites commanding all ground in common range need not be so high as those on lower ground commanded by neighboring heights. The principles laid down and the reasons expressed for the normal profile apply equally well to all itn variations. CHAPTER IV. THE TRACE OF A FIELD FOliTIFICATION". 33. Trace.— The term, trace, is used by milita/y engineers to denote the plan, or the general outline, of a field fortification upon the ground. (Art. 11.) In field fortifications the governing or principal line used constructing the trace is the projection upon the plane of site, of the line from which the fire is deliv- ered, viz, the interior crest. The projection of the interior crest upon the ground is called the sub-crest. 34. Kinds of fire. — The trace of a field for- tification can not be marked intelligently upon the ground, until the positions which the enemy may possibly occupy, and the kinds of fire he can bring to bear, are known. Different names are used to designate the fire, both of artillery and musketry, depending upon its direction and kind. The terms, front, reverse, flank, and cross fires are used to designate the direction of the fire with respect to the line aimed at. The projectile striking the line in front, or in rear, or at its ex- tremities, or crossing in its flight other projectiles TRACE OF A FIELD FORTIFICATION. 35 coming from u different direction, gives tlie name to the kind of fire to which the line is exposed. When the direction of the fire is perpendicular, or nearly so, to the line aimed at, tlie fire is a direct une ; if this direction makes an angle with the line tiimed at, it is oblique; if this angle is very slight, it is a slant fire ; if no angle is made but the di- rection coincides with the prolongation of the line aimed at, it is an enfilading fire ; if it makes no angle, but is in front of the line, it is called a flanking fire ; etc. A line of troops or a line of parapet may be exposed to a front direct fire, a reverse fire, a slant fire, a slant reverse fire, an enfilading fire, a cross fire, etc. A column of troops might be exposed to a front direct, an oblique, a flank, a reverse, or a cross fire, ac- cording to the directions from which the projectiles came. Other desisrnations are used to denote the hinds of o fire. Thus, a direct or pitching fire means one in which the projectile is fired from a gun at ordinary elevations, and with the service charge ; a curved fire, when the angle of elevation is greater than usual, and the amount of charge used in the gun is less ; a vertical fire, when the angles of elevation are still larger. Other designations denote the kind of fire, as de- termined by the position of the line of fire with re- 36 FIELD FORTIFICATIOKS, spect to the surface aimed at, or some peculiar char acteristic which marks it. Thus, a ricochet tire, is the result produced by small angles, low charges, and spherical shot ; a grazing fire is when the projectile passes very near the surface ; a plunging fire, when the projectile comes from a higher level than that occupied by the object struck ; etc. 35. Salients and re-entrants. — An assailant when attacking a field work naturally tries to advance over that ground which offers the least obstruction to his free movements, and which does not expose him to the fire of the defenders. In the latter case, the most favorable ground will be that upon which the defence cannot bring a fire, and the most unfavorable will be that upon which the defence can bring a cross fire. In order to have an effective direct fire upon the ground exterior to the parapet, the interior crest should be perpendicular, or nearly so, to the direction in which the fire is to be thrown. And since there are several directions in which the fire is wanted, it follows that the interior crest must be a broken line. Those angles of the interior crest which project outwards, and towards the enemy, are called salient angles ; those projecting inwards, are called re-en- tering angles. 36.— General principles. The following gen- TRACE OF A FIELD FORTIFICATION. 37 eral principles should govern the engineer in the se- lection of the trace. 1. The trace should be as simple as possible. 2. The direction of the interior crest should be such as to admit of bringing a strong direct fire upon the ground liable to be occupied by the enemy. 3. The dimensions of the works and interior space should be proportioned to the number of men in- tended to defend them. 4. The principal lines of a fortification should be given such directions that they cannot be easily en- filaded, nor seen in reverse. 5. No salient angle should be less than 60°. 37. That the trace should be simple is evident Field fortifications are works which are to be con- structed in a short time and under pressing circum- stances. Multiplicity of details and refinements of construction would cause a waste of valuable time, a thing of far greater importance in a campaign than theoretical perfection. A strong direct fire is obtained, as previously stated, by placing the line of the interior crest as nearly perpendicular as possible to the direction in which the fire is to be delivered. A small deviation is admissible, depending upon the advantages to be derived from changing the position of the line. The work should be proportioned so as not to re- quire more men to defend it than are available. A 38 FIELD FORTIFICATIONS, fortification is a passive defence, and is an inert and helpless factor, if not defended by a live and active force. It is not always practicable to arrange the lines so as to satisfy the second and the fourth conditions at the same time. To comply with the former would be, in many cases, to expose the line to the fire con- demned by the fourth condition, The fourth con- dition must, under such circumstances, be satisfied by some expedient which will annul the effects of the enfilade fire. The expedients generally used are tra- verses. Their employment and construction will be explained hereafter. Angles of less than 60°, for salients, restrict the interior space, and do not leave sufficient room to serve the guns which may be used in the salients ; they also weaken the earth work of the parapet at these points. It may not be possible to have the trace fulfil all these conditions. If it does, and if the profile of the parapet is a strong one, the fortification will be a good one and will prove a formidable obstacle in the enemy's way. CHAPTER V. : FIELD WORKS. 38. Field Tvorks and lines. — Field fortifica- tions are usually divided, according to the extent of the position fortified, into two general classes, viz. : field works, and lines. The term, field work, is applied to the temporary fortification used to strengthen a position of limited extent, and in which the troops occupying it are ex- pected to be dependent upon themselves alone for a successful resistance to an assault. The term, line, is applied to the temporary for- tification, or chain of fortifications, which is used to strengthen a position of considerable extent, and is to be defended by an army or a large body of troops. These two classes of fortification do not differ as to the details of their construction. 39. Classes of field works. — The trace of a field work depends u})on the directions in which it has to fire. (Art, 34.) It may have to fire on all sides of the position ; or over a limited portion of a circle ; or in special directions only, according to the posi- tions which may be occupied by the enemy's artil- 40 FIELD FORTIFICATIONS. lery. These different circumstances give rise to three kinds of field works, viz. : 1. Field works exposed to the artillery fire of the enemy in one direction only, or in front ; 2 Field works exposed to artillery fire of the enemy on the flanks as well as in front ; and 3. Field works exposed to this fire upon all sides of the position. It is evident that in the trace of a work of the first kind, the position which the enemy may have in its front need only be considered. The trace of a work of the second kind must be considered under the supposition that the enemy may appear in front and on the flanks, that is in a por- tion bounded by a part of a circle. The trace of a work of the third kind must be considered under the supposition that the enemy may appear on all sides of the work. It is plain that parapets are needed on all sides, in the works of the third kind, to shelter the men from the enemy's fire ; and that they are only needed on the exposed sides, in works of the first and second kinds. The works of the first and second kinds may then be left open on the sides not exposed to artil- lery fire, or they may be closed by some obstruction. From the construction of the parapets, in these dif- ferent cases, there arise three classes of field works, viz. : open, half-closed, and closed works. FIELD WORKS. 41 40. Open, and half-closed works.— A simple straight line of parapet is an example of an open field work. It is plain that a work of this kind would only be used where it would be impossible for the enemy to get on the flanks; or around in its rear; or where i fc is intended to abandon it as soon as the enemy gets oil the flank or in the rear. Half-closed works are used where there is any dan- ger of the enemy appearing suddenly upon the flanks. They are closed by some obstruction so as to prevent a surprise or sudden attack by small bodies of infantry that might appear suddenly in rear of the position. One of the simplest forms of the half-closed field work is the redan, (Fig. 3.) It Fig. 3, A consists of two strais^ht lines A-i-A forming an angle BAG, which / ! \ is pointed towards the enemy. / \ The angle at A is called the ^_ [ .A^ salient ; the sides B A aiid C A in'e called the faces ; the line B C is termed the gorge ; the line A D is called the capital. The sides of the redan are usually from thirty to sixty yards long. When' the length is less than thirty yards it is called a fl^che. The redan, as here shown, delivers its fire over a part of a circle but has no front fire. If it he necessary to have a fire in the direction of the capital, it may bo obtained by using the blunted redan, which is con- 42 FIELD FOR TIFICA TIONS, strncted by stopping the faces at points, as E ^'i^ F, and connecting these points by a straight parapet. Two redans are sometimes placed side by side and joined to each other, making a work known as the double redan ; sometimes the outer faces of the double redan are made much longer than the faces which are connected, in Avhich case the work re- ceives the name of priest-cap, or swallcw-tail. 41. Lunettes. — If lines of parapet which are paral- lel, or nearly so, to the capital, are added to a redan, the construction will be that known as a lunette, (Fig. 4.) These parts, B C '^^i^^ D E which were added to the redan, BAD, are termed the flanks ; tlio angles, at B ^i^^id at D '^i"6 E culled shoulder - angles. A lunette is therefore a field work consisting of two faces and two flanks. 42. Bastioned Front. — Suppose two lunettes, F Fig. 5. c A and B to be connected by a straight line of parapet, C D (Fig. 5.) The resulting construction is called a FIELD WORKS. 43 bastioned trace, and the portion between the capi tals through E ^ind F a bastioned front. The lun- ettes when thus joined are called bastions ; the line of parapet joining thera is the curtain ; and the re- entering angles ac C and D are known as the cur- tain angles. 43. Closed works. — Since the works of this kind are exposed to the enemy's fire of artillery on all sides, the position must be entirely surrounded by a parapet. The trace of such a work may be of any form, either circular, square, quadrilateral, polygonal — regular or irregular — all salient angles, or some salient and some reentering angles. A closed work in which all the angles are salients is called a redoubt ; if there are reentering angles, it is termed a fort. 44. Redoubts. — A redoubt may be of any figure provided it has no reentering angles. Circular re- doubts are sometimes used, but are objected to on account of the divergence of their fire. Redoubts are usually polygonal in plan, and, on horizontal sites, there is no reason why the plan should not be a regu- Fig. 6. lar figure. The most simple and the most usually employed tigure is the square, (A Fig. 6). 44 FIELD FORTIFICATIONS. The redoubt, B (Fig. 6) is a form which is fre- quently used when it is desirable to combine its fire to the front with that coming from other points to the right and left of it, and still keep a direct fire on the flanks. The advantages claimed for redoubts over forts are simplicity of trace, ease of adaptation to irregu- lar sites, less labor required in their construction, and fewer men are necessary to defend them for the same amount of space enclosed. 45. Star forts. — A fort of this class receives its name from the general resemblance which its trace has to the conventional symbol used to represent a star. The star of six, or of eight, points is the kind mostly employed, although any number of points may be used, being determined ordinarily by the confor- mation of the ground on which it is built. The construction of the trace of a six pointed star is as follows : Describe an equilateral triangle ; trisect its sides ; on the middle por- tion of each side, determined by this trisection, construct an equilateral triangle. The resulting construc- tion gives the six-pointed star, as shown by the full lines in Fig. 7. The trace of the eight-pointed star is constructed in a similar manner, using a square instead of the FIELD WORKS, 45 equilateral triangle as the preliminary figure. The construction is as follows : Describe a square ; trisect its sides, and construct equilateral triangles on the middle portions thus de- termined (Fig. 8) ; or, what is a better trace, replace the right angles of the square by angles of 60', as shown at A in the lower half of Fig. 8. 46. Bastioned fort. — If the space to be enclosed is circumscribed by a poly- gon, and on each of the sides a bastioned front (Fig. 6) is constructed, the resulting work is called a bastioned fort. The polygon used to enclose the space to be de- fended may have any number of sides, and may be either a regular or an irregular figure. On a hori- zontal site, there is no reason why the figure may not be a regular polygon. On a level site, the polygon generally used is a square unless a larger work is required. The construction of the trace, when the circum- scribing polygon is a square, is as follows : Let A B (^ig- 9) be one of the sides of the cir- cumscribing s/iuare. Bisect this side and at its mid- dle point, C erect a perpendicular, C P '^^^ l^^y ofi a distance C P equal to one-eighth of A Bi J^i" 46 FIELD FOR TIFWA TIOJVS. the point P thus determined with the ends, A and B, of the side of the square. Lay off from A ^^^ B, on the lines A P, and B P, the distances A G and B H, each equal to two-sevenths of A B. Draw through G the line G F, so that the angle G F B shall not be less than 90°, nor greater than 110°, and produce it until it intersects the line B P P^'o- longed. Do the same at H, so that the angle H D A shall fall within these limits just named. Join the points F and D thus determined by a straight line. The line A G F D H B will be the trace of a bastioned front constructed upon the side A B of the circum- scribing square. Do the same for the other three FIELD WORKS. 47 sides of the square, and the resulting construction is the trace of a bastioned fort. In a simihir "hianner the construction is made for a bastioned front, when the polygon has a greater number of sides than four. The only variation made is in the length of the distance C P, this distance being taken one-seventh of the side of the polygon instead of one-eighth, when the circumscribing poly- gon is a pentagon ; and one-sixth, when the polygon is a hexagon or a polygon of a greater number of sides. It is evident that the triangle can not be used for the circumscribing polygon. If it were, the fifth con- dition for field works (Art. 3G) could not be satisfied. 47. Nomenclature. — The side of the circum- scribing polygon is called tlie exterior side ; the line, C P, is called the perpendicular ; the lines D A and F B. tlie lines of defence ; the angles G A C Jiiid H B C are culled the diminished ang- les. The other parts are named as already stated in articles 40, 41, and 42. 48. Sectors -without fire, and dead spaces. — The star and bastioned forts were devised to remedy the defects of sectors without fire, and of dead spaces, which are found in redoubts. A sector without fire is the name given to that space exterior to a work which is not defended by the direct fiic of the adjacent faces. Thua the faces A B ^^^ A C o^ the redan (Fig. 3) or th« 48 FIELD FORTIFICATIONS. faces A B and AD of the lunette (Fig. 4) if prolonged, would include a space not swept by a direct fire from the adjacent faces. This space is in a great measure un- defended, except by oblique fire, so far as the faces are concerned. The space included between lines drawn through a saUent, perpendicular to the faces, is known as the sector without fire. Any ground, over which the defenders' fire may pass, but so high above the assailant that he can not be injured by it, is called, a dead space, or a dead angle. Thus, in the redan or the lunette just men- tioned, if the enemy gets into the ditch, he is safe from any fire coming over the parapet. The ditch jn each of these cases has the defect known as a dead r space. It was to remedy these defects, which exist in all works where the angles are all salients, that parts of a work were drawn back, or made to form re- entering angles, for the purpose of arranging lines from which direct fires could be brought to bear upon the ground not defended by the lines adjacent to it. 49. Flanked disposition. The arrangement of lines by which direct fires are brought to bear upon sectors without fire, and by means of which dead spaces are reached by a fire from the parapet, is called a flanked disposition. This term is used to de- note this arrangement, because the enemy advancing upon a salient, where this arrangemen;t is used, is FIELD WORKS. 49 exposed to a flanking fire— a fire parallel to and in front of the line attacked. (Art. 34.) The defects of a flanked disposition are : the ex- posure of the lines of the work to enfilading or re verse fires ; a contraction of the space enclosed by the work ; and a partial sacrifice of the strong direct fire which the work might otherwise have. A convergence or a crossing of fires upon the ground, which can be obtained by a flanked disposi- tion at certain points, is productive of demoralization, as well as severe loss, among troops exposed to it. For this reason, a flanked disposition was formerly laid down as an essential element to be incorporated in all schemes of defence. It is still an important factor. But, in the use of the long range weapons now employed, the strong direct and front fire u considered to be so important, that it is thought best not to sacrifice any part of it, if it can be avoided. Especially is this so when the advantage to be gained is accompanied by the defect of ex posing some of the lines of the work to an enfilading fire. An important advantage claimed for the flankeo disposition is its ability to sweep its own ditches by fires from the work itself. In field works, the ditches are too narrow, and their depth too slight, to make this a matter of great importance, when compared with the defects which accompany the flanked dispositiou \: 50 FIELD FORTIFICATIONS, Still, in field works, especially in the case of a large independent work, or a work which occupies the key point of an important position, the ditches should not be left undefended. They may be flanked by inde- pendent defences placed in the ditch — an expedient which will be explained hereafter. 50. Relation between the parts. — An in- timate relation exists between the different parts of a flanked disposition. These are so connected that a change in any one affects more or less all the others. The flanks are arranged to cross their fires in front of the salients, and to remove the dead spaces in the ditches (Fig. 9). To remove the dead space in the ditch in front of the curtain, requires that the fires from the flanks should reach the bottom of the ditch at the middle point of the curtain. To cross the fires effectively in front of the salients, requires that the line of defence should not be longer than the close and effective range of the weapon used to arm the flanks. It is plain that the length of the exterior side must depend directly upon the length of this line of defence, and that the length of the curtain, which must admit of the ditch being flanked, must depend upon the relief of the interior crest of the flanks, and the inclination of the superior slope. The shorter the line of defence can be made, at the same time keeping the curtain long enough to al- low its ditch to be swept by the fire of the flanks. FIELD WORKS. 51 the more effective will be the cross fires in front of the salients, the longer will the eneray be under this cross fire, and the greater will be the chances of repulsing any assault made by him. 51. Least and greatest exterior sides of a bastioned front. — Taking the height of parapet at eight feet and the depth of the ditch at six feet, this relief of fourteen feet will be the least used ; tak- ing the height at twelve feet, and the depth at twelve, the corresponding relief of twenty-four feet will be the greatest used. Assuming the superior slope at -J-, the least length of curtain for the least relief is fifty-six yards ; the least length for the greatest relief is ninety-six yards. Using the construction given in Art. 46, it will be seen that for a curtain of fifty-six yards in length, the exterior side must be about 125 yards long ; for the curtain of ninety-six yards, the exterior side must be two hundred and fifty yards long. The least length of the exterior side will there- fore be between one hundred and twenty-five, and two hundred and fifty yards, depending upon the re- lief of the work. The greatest length of the oxtvrior side depends upon the length given to the line of defence. If the weapon used to arm the flanks is the rifled mus- ket — the weapon now used by infantry — its close and effective range determines the length of the line o. 52 FIELD FORTIFICATIONS. defence. The limit of accurate aim for the ordinary soldier is about three hundred yards. Assuming thia to be the length of the line of defence, the exterior side will be about four hundred yards long, and will be the greatest length ordinarily used. 52. Defects of a bastioned fort. — The bas- tioned fort can not be used upon an irregular site, without sacrificing some of its most important quali- ties. It requires, also, considerable time and labor to build it. For these reasons it is rarely used as a field work, except when it is to be an independent work, and used to defend some important point. One point in the trace of the bastioned fort re- quires particular attention, and that is the counter- scarp of the ditch. If the counterscarp is kept par- allel to the interior crest, there will be a dead space in the ditch near the shoulder angle, and in the ditches in front of the flanks, unless a modi- fication is made either in the profile or in the width of the ditch. The modifications usually made are of three kinds, and are as follows : One is to widen the ditch in front of the curtain by removing all the earth included between the cur- tain, the flanks, and the counterscarps of the faces prolonged. This method removes the dead space en- tirely. FIELD WORKS. 53 Another is to widen the ditch ut the top only, by giving the counterscarp of the ditch in front of the flanks and on the prolongation of the faces, a slope which will expose the bottom of the ditch in front of the flanks to the fire from the flank opposite. This method removes the dead space. A third way is to slope the counterscarp of the prolongation of the ditch in front of the faces, so that the bottom of this ditch in front of the shoulder angles shall be swept by the fire of the flanks. This method removes the dead space in the ditches of the faces, but does not remove those of the flanks. The first method is the one used in permanent fortifications. The amount of time and labor required to make this modification forbids its use in field works. The second method has the same defects, but not to so great a degree. The third method, though partial in it effect, is the one generally adopted in field-fortifications. 53. Defects of the star fort. — Star forts are better adapted to irregular ground than the bastioned forts. Otherwise, they possess all the defects of the latter, without the complete flanking arrangements which characterize the bastioned system. CHAPTER VI. LINES 54. Classes of lines. — The field works known as lines are divided into classes, according to the object for which they are constructed ; or, according to some peculiar arrangement of their parts, or other charac- teristic quality. Some of the most commonly known are the lines of circumvallation • of counter- vallation | intrenched camps | single and multiple lines ; continued lines ■ lines with intervals; etc. The classification of lines into continued lines and lines with intervals is the one which is used in this chapter. 55. Continued lines. — When the entire front to be defended is covered by a co7itinuous line of para- pet, the work is called a continued line. There are no openings in a continued line, except those made for the use of the defence. 56. Lines with intervals. — When the front to be defended is covered by a number of field works, scattered along this front, and placed near enough to- gether, to sweep the intervals with their fire, the whole arrangement forms a disposition called a line LINES. 55 with intervals. — Field works placed so near to each other, that each one can bring its fire to bear upon the ground in front of tliose adjacent, are said to be in defensive relations with each otljer. I. Continued Lines. 67. Kinds of continued lines. — The principal types of continued lines in use are the straight line, the tenaille line, the redan, the indented, and the bastioned lines, which are easily distinguished from each other by their traces. 58. Tenaille line. The trace of the tenaille line is formed by making the alternate angles salient and re-entering ; the condition being imposed that the p Fig. lOa re-entering angles must not be less than 90% nor greater than 120" (Fig. 10). The line A B, joining the extreme right and left 56 FIELD FORTIFICATIONS. salients, is calkul tlie front ; and tlie ground in front of it, within range of the fire from the line, is called the zone of defence. The greatest length of the faces depends upon the range of the weapon used in its defence. The main object of the tenaille line is to obtain a cross fire upon the zone of defence. This cross fire, to be effective, must be within the range of accurate aim of the wea- pon used. To find the greatest length for the faces, the fol- lowing construction may be used. Assume the angles lor the salients, which must be known. Take the salient at A? and produce the face p A, until the dis- tance A P> is equal in length to the range assumed. Through this point, P, draw a line P C parallel to the capital A H» and produce it until it intersects A B? the front of the line. Its point of intersection with this line will determine the position of the salient adjacent to A- Since the re-entering angle is not to be less than 90°, the greatest length of the faces, when the positions of the salients are fixed, will be when the re-entering angle is 90°. For angles greater the length of the faces will be less. The least length of face is taken at thirty yards, so that there shall be no dead spaces in the ditch at and near the salients. The more obtuse the salient angles are made, the more difficult it will be for the enemy to enfilade LTNES. 67 the. faces. This exposure to enfilading fire, and the amount of labor required to construct the line, are the principal objections to its use. 59. Redan line. — A series of redans, placed at intervals and connected by straight lines of parapet, (Fig. 11) or by lines with very slight re-entrants, or salients, is termed a redan line. Fig. II. The traces of the redans are usually made equila- teral triangles. The salients are placed about two hundred and forty yards apart, and the faces are made sixty yards long. This line is sometimes known as Vauban's line. There is another redan line composed of large and small redans, like that shown in Fig. 12. It is Fig. 12. known as a continued redan line, to distinguish it from Vauban's line. 60. Indented lines. — The indented, or cr0- maill^re line consists of short and long branches^ which may be arranged as represented in Fig. 13. The long branches are usually made seventy yards long, or more, and are directed towards ground which 5g PIELD FOR TtFICA TIONS. cannot be occupied by an enemy. The short branches are made about thirty yards long, and are used to Fig. 13. flank the long branches. Instead of giving the long branches directions parallel to each other, as indicated in Fig. 13, they may all be directed upon a single point, which the enemy cannot reach, as shawn in Fig. 14. Fig. 14. 61. Bastioned line. A bastioned line is com- posed of a series of bastioned fronts, joined to each other on the same general line, (Fig. 15) the salients Fig. 15. being placed from two hundred and fifty to four hun- dred yards apart. The line joining any two consecu- tive salients is taken as an exterior side,- and a bas- tioned front constructed upon it by the rules already given. 62. Advantages and disadvantages of con- tinued lines. — Continued lines have been much LiifEs. 69 used in past military operations, and will, in all prob- ability, be" used in the operations of the future. Continued lines may be usefully employed where a passive defence only is to be made, and where the position to be defended is limited in extent, and not exposed to flank attacks. They are not fitted for an active defence, and they have the serious disadvantage of being untenable, when any part of the line has been taken by the enemy. They require a large amount of labor to construct ; and it is a very doubtful question, whether the ad- vantages they give compensate for the time and labor employed in their construction. II. Lines with Intervals. 63. Lines "with intervals. — This class of lines differs from the continued lines, by leaving intervals along the front of the position, which intervals pre- sent no obstructions to an enemy moving through them, excepting so far as they may be defended by the fire of the works, or may be obstructed by natural obstacles, or by artificial ones placed along the front. The works forming the line may be placed so close to each other, as to be in defensive relations; or they may be so far apart, as to admit only of their de- fending the intervals between them. If the works forming the line are to be in defen- sive relations, that is, if they are to afford a reciprocal 60 PIELD FORTimCATlONS. defence, it is evident that the remarks made upon obtaining the length of the line of defence, in a flanked disposition, apply equally to determining the distance between the works. Thus, a line of redoubts (Fig. 16) placed so that their salients are three hun- dred yards apai't, along the line, A B^ would have the works on this line in defensive relations with each other, the weapon used being the musket. If it be required to defend only the intervals be- tween them with the musketry fire, this distance be- tween the salients of the consecutive works might be as much as six hundred yards, as this would bring the middle portion of the interval within the range of accurate aim. If the defence is to be made with artillery, the distance, in the first case, might be one thousand, or even so great as one thousand five hundred, yards. In the second case,, these distances may be doubled. If an obstinate defence is to be made, a second line should be used (Fig. 16). A second line is es- pecially useful when the works on the outei line are not in defensive relations with each other. • LINES. 61 The second line should be placed behind the first, and distant from it, about one-fourtli, and cer- tainly not greater than one-half, of the distance be- tween the works in the first line. When artillery is used ir. the second line, a good position would be about six hundred yards distant from the first. This places the second line just out of range of aimed musketry fire, but in close range of artillery fire. 64. A third line of field works is sometimes built. The general arrangement of the works of this third line, with the works of the first and second, conforms to the principles employed in arranging the works in the second line (Fig. 17). A third line might be useful, in case of an active defence, since the works placed along tliis line can be utilized as screens for the reserves and for bodies of cavalry. A fourth line would evidently be of no practical service in the defence of a position. A force, unable 62 FIELD FORTIFICATIONS, to retain possession of the outer lines, could not be expected to hold the fourth. The number of lines, whether a single one, or two, or three in number ; the kinds of works to be used on each line ; the distance apart of the works on each line ; the distance apart of the lines ; and the details of their construction, depend upon the natural fea- tures of the ground, the numbers and kinds of troops which are to occupy them, the range of the arm used in their defence, and the time disposable in which to construct them. 65. Advantages and defects of lines with intervals. — Certain advantages are claimed for lines with intervals. These advantages may be briefly stated to be as follows : 1. The lines with intervals admit of either passive or active defence. They are peculiarly fitted for the latter. 2. Lines with intervals are more easily adapted to the natural features of the ground than continued lines. 3. For the same extent of front, they require fewer men to defend them, and the works require less labor to construct, than other kinds of lines. 4. They admit of a better use being made of raw and inexperienced troops. 5. A line with intervals may still be defended, even after a part of the line has been captured, or after the enemy has broken through the line. LINES. 63 The main defect inherent in lines of this class, is the sub-division of the defenders into several independ- ent commands, by which unity of action of the whole command is impaired. This defect is reduced somewhat by a proper dis- position of the works. A few capacious and strong works are better than a large number of small ones. Experience has shown that a body of one thousand men, in a single well-constructed work will offer a more effective resistance to the attacks of an enemy than tlie same number would, when scattered among three, four, or more, smaller works. The diflBculty which a gen- eral would meet, in obtaining experienced officers fit for these independent commands, and in having these officers to act in unison with each other, gives sufficient grounds for such a result to be expected under ordin- ary circumstances. CHAPTER VII. THE SIZE OF A FIELD WORK, THE NUMBER OF THE GARRISON, AND THE SELECTION OF A TRACE. 66. Size. — The size of a field work, when built upon a level site, depends principally upon the number of men intended for its defence. When built upon an irregular site, the size, in addition, depends fre- quently upon the extent of ground to be swept by the fire of the work. A good defence of a work is obtained only by a strong musketry fire, which is only obtained by allow- ing a musket for each pace, measured on the interior crest. The number of men to form a single rank and to furnish this fire would be equal to the number of paces contained in the length of the interior crest. It will be sufliciently accurate to assume four paces for each three yards in length. An interior crest of three hundred yards would require four hundred men to line it in single rank. A vigorous defence requires not only men enough to line the parapet, but a good many more to supply the vacancies from casualties or other causes, and to furnish support to the line. Double the number required to man the parapet is considered to be sufficient. That THE SIZE OF A FIELD WORK. 65 is, a vigorous defence of three hundred yards of inte- rior crest would require a total of eight hundred men, Hence, to determine what the length of interior crest shall be, the number of men being known, it will only be necessary to divide tlie number of men by two, and take this quotient for the number of paces which the interior crest should have. Conversely, the length of interior crest being known, it will only be necessary to find the number of paces in its length, and double this number for tlie number of men which will be required. 67. Garrison. — A body of troops stationed in or near a field work, to defend it, is called its garri- son. The garrison of a work, when it is practicable, should always be a complete organization, or composed of detachments belonging to the same unit of force. Garrisons should not live within field works, un- less there is a pressing necessity for this to be done. As a rule, they should encamp near the works they are to defend, and only keep guards within the works. Nevertheless the engineer, or other officer, who lays out a field work should always consider the pos- sibility of its being occupied by a garrison, and should provide the necessary accommodations, so far as interior space may be required. The amount of interior space to be enclosed be- 66 FIELD FORTIFICATIONS, comes a matter of importance under these circum- stances, and especially in redoubts. 68. Enclosed space. — The amount of space to be enclosed by a redoubt can be easily calculated. It is usual to allow from one and a half to two square yards of space for each man ; a space of sev- enty-five square yards for each field gun ; a space of twenty square yards for a powder magazine ; and a space of one hundred and seventy-five square yards for a traverse which is used to protect the outlet, or passage way, leading out of the work. Suppose a field work is to be built, which is to be a square redoubt, to be armed by four field guns, and to have a traverse opposite the outlet ; what is the area to be enclosed, and the length of interior crest, the garrison numbering four hundred men ? From the quantities above, the space for the ac- commodation of the men, etc., will be as follows : 400 men, allowed 2 square yards each, 800 square yards. 4 field guns, " 75 " " 1 powder magazine, 1 traverse. Total 1295 " " In addition to this space, there must be room for the part of the parapet between the interior ci-est and the banquette slope. The distance from the foot of 300 a (( 20 a (6 175 ee (( THE SIZE OF A FIE LI) WORK. 0? the banquette slope to the sub-crest, in the ordi- nary profile, is three yards. This distance must be enclosed, as necessary for the troops. The square root of 1295 may be taken to be thirty-gix. A square whose sides are each thirty-six yards long will enclose a space sufficient for the accommo- dation of the men and guns, and allow for a traverse and a magazine. If a square be taken whose sides are six yards longer, that is, each side equal to forty- two yards in length, it will enclose a space sufficient for the men, guns, etc. ; and will also allow room for the parapet. A square redoubt whose sides are forty-two yards ong, or whose entire interior crest is one hundred and sixty-eight yards long, will enclose space enough to accommodate a garrison of four hundred men, four guns, etc. The rule given in Art. QtQy would require a force of four hundred and forty-eight men to defend this work. It is evident, then, that there must be a least length of interior crest, which being defended accord- ing to the rules laid down, shall enclose the necessary space for the accomodation of the garrison. 69. Relation between least length of interior crest, and space enclosed for a square redoubt for a given number of men, etc. 68 FIELD FORTIFICATIONS. Let X represent the number of yards in the interior crest of one of the sides of the redoubt ; n, the number of men forming the garrison ; 5', the number of square yards allowed for the guns, magazine, and traverse. Let it be supposed that each man is allowed two square yards, and that for every three yards of interior . crest, there shall be eight men. These conditions may be expressed by the follow- ing equations. (aj-6)« = 271 + «' (1) iw = |a; X 4 (2) If the redoubt is to be armed with field guns, and to have a powder magazine and a traverse, the value of s' may be determined by the rule already given. Suppose the redoubt is to be armed with four field guns, etc., and that the space allowed for them is that named in the last article. After substituting for s, its value, four hundred and ninety-five square yards, equations (1) and (2) may then be combined and solved with respect to x and n. Their values will be found to be X = 44, and n = 473. If the redoubt is not to be armed with artillery, but is to have a traverse opposite the outlet, the value of s' will be one hundred and seventv five SELECTIUX OF THE TRACE. G9 yards, and the corresponding ^alues for x und ?t, will be X = 37, and ti = 395. All square redoubts in which the lengths of the sides are less than thirty-seven yards and forty-four yards, will not have the requisite space enclosed. Since the area increases in a much more rapid ratio than the perimeter, all square redoubts whose sides are longer than those named, will contain more space than is absolutely suflBcient. 70. Selection of the trace. The selection of the trace of a field work is dependent upon the num- ber of men that can be spared to defend it, and upon the time that can be allowed in which to build it. The selection is also dependent upon the natural features of the site. Thus, suppose a field work is to be placed in front of a bridge, to defend the crossing of the river at that point (Figs. 18 and 19). The Fig. 18. Fig. 19. redan, which was a fitting trace where its site is in the bend of the river (Fig. 18) would not be so well fitted for the defence as a lunette would be, if the site is bounded by a straight course of river (Fig. 19.) The selection is also infiuenced by the particular 70 FIELD FORTIFICATIONS. object to be attained. It is plain that a trace which would bring a strong direct fire upon ground which an enemy would not occupy, would be a faulty one. The arrangement of the trace upon the site, after the trace has been chosen, is an important matter, and may be considered as a part of the selection. Thus, a field work to be built in rear of a marsh, the crossing of which is impracticable for the enemy (Fig. 20) would have a faulty trace, if its lines were H BO placed as to bring strong direct fires upon the marshy ground, and not upon those portions along which the enemy would be obliged to approach in his advance upon the work. The same fault would exist where the trace was assumed, if its lines were not properly directed. Thus, suppose a square redoubt was to be placed in rear of the marsh, it is plain that the redoubt B (Fig. 20) would have the defect just named, while the redoubt A would not. If, instead of a marsh, the ground had been high, SE LECTIO A OF THE TRACE. 71 it is easily seen that the redoubt at A i* better placed than the one at Bi No absolute rules can be laid down which will apply to all cases. The engineer must exercise his skill and ingenuity in eacli case, taking care to harmonize as far as possible the conditions to be observed, whenever they conflict. CHAPTER VIII. CONSTRUCTION OF FIELD WORKS. 71. Tracing. — After having determined upon the dimensions of the profile, and having selected the trace, the first step in building the work is to mark its out- line upon the ground. This operation is called trac- ing. The operation of tracing consists in marking the sub-crest and other necessary lines upon the ground, so that they can be distinctly seen, and can be used to determine all other lines and points of the work. The " tracing " of a field work may be made by means of a drawing which represents the work to be constructed ; or it may be made directly upon the ground without the use of a drawing. When a drawing is used, it is usual to take two points upon this drawing, which can be easily located upon the ground, and join these points by a straight line. This line is then taken to represent a base line which can be used in laying out the work. From the different angular and important points of the plan, perpendicular lines are drawn to this base line. The distances to the base line, and the distances in- tercepted n})on it, are measured and noted upon tlie drawing. Going to the site, the two assumed points CONSTRUCTION OF FIELD WORKS. 73 are located upon the ground, and a straight line ia drawn through them. This line is the base, and is marked upon the ground either by a cord, or by a furrow in the ground made with a pick. The distances are then measured off on the base line, and the points marked where the perpendiculars are to be con- structed. These perpendiculars are then constructed by some of the simple methods used for this purpose, and the distances to the angular points laid off on these. These points, thus determined, are then marked by stout pickets driven into the ground. The angular points are then joined by straight lines, and these lines are then marked upon the ground by a cord stretched between the adjacent points, or by a furrow made in the ground. The marking of these lines completes the tracing of the sub-crest. If there is no drawing, but tlie work is to be laid out directly upon the ground chosen, it is usual for the engineer to first select the salient points of the work, and mark them by stout pickets driven into the ground. He may then determine the reentrants by inspection, or by some rule. Which method he will use will depend upon the kind of tracing to be made. The angular points, all determined, are then connected by straight lines, marked as in the previous case. 72. Profiling. — The operation called profiling forms tlie next 'step in the construction. This opera- 74 FIELD FORTIFICATIONS. tion consists in erecting, at proper points along the sub- crest, wooden profiles which give the form of the parapet at those points, and which guide the workmen in the construction of the work. Profiles are placed at the extremities of a parapet ; at points along the sub-crest from twenty to thirty yards apart ; at the salients and reentrants ; and at any jioints where a change is to be made in the dimensions, or in the direction of the parapet. The profiles are made perpendicular to the subcrest, excepting at the angles, where they are made to bisect the angles. An ordinary profile may be constructed as follows : A straight line, as D' K (Fig. 21.) is drawn on the Fig. 21. ground at D '<^^^ perpendicular to the sub-crest. A distance, D' E'> equal to the thickness of the parapet, is laid off and marked by stout pickets driven into the ground, at D' ^i^d. £'• Poles or strips are nailed to these pickets in a vertical position, and held firmly in place by a strip, H H ? fastened to them. It will be found convenient to make this strip horizontal, which may be done by a level. CONSTRUCTION OF FIELD WORKS. 75 The height of the interior crest is marked on the pole at D'j and the exterior crest on that at E'* A wooden strip, from two to three inches wide, and one ihch thick, is then nailed to these poles at the points D and E» jiist marked. These wooden strips may be made by sawing boards into pieces of the necessary width. A distance H' F is laid off on the horizontal strip H W i equal to the base of the slope wliich the earth is to take. A stake is driven at this point, and the point p marked upon it. A strip, E F? extended until it meets the ground, is then nailed to the up- rights, which strip, in this position, marks the inclina- tion of the exterior slope. Similar strips are fastened to the other uprights to mark the inclination of the interior slope, the width of the banquette, and the inclination of the banquette slope. The profiles being finished, the foot of the banquette slope and the foot of the exterior slope are marked upon the ground in a way similar to that used for the sub-crest. This operation completes the profiling and tracing of the parapet. The profile of the parapet being finished, the points G «iii<^ K- ^1* points of the scarp and counter- Bcarp, are marked by pickets driven into the ground. The crests of the scarp and counterscarp are then traced upon the ground. Where attention is to be paid to the slopes of the ditch, short strips are nailed to 76 FIELD FORTIFICATIONS. the pickets, marking the inclinations to be given. In addition, the points L '^nd M are determined, and the lines projected in these points are traced upon the ground by means of a pick. 73. Execution of the "work. — The third step is begun by bringing the working parties on the ground and commencing the excavation. The first thing to be done is to remove all brush, trees, rocks, or obstructions, which may hinder the work of excavation. In this kind of work, as well as in the excavating, the working parties are divided into smaller bodies, generally termed gangs. The gangs, being furnished with shovels and pick- axes, are distributed along the line of the ditch, under the supervison of competent foremen. It is recommended to divide the crest of the coun- terscarp into lengths of six feet or more (sometimes twelve) and to mark the divisions by small pickets. The sub-crest is then divided into the same number of divisions, which are connected with those already niarked upon the counterscarp, by furrows traced upon the ground, tliese furrows being prolonged until they intersect tlie foot of the banquette slope. In this way, there are formed small areas, in each of which a work- ing party can be placed, and in each of which there will be room for the party to work without crowding. The number of workmen, the proportion of picks and CONSTRUCTION OF FIELD WORKS, "J"? shovelg, and the positions of tlie men, will depend upon the nature of the ground to be excavated and the dis- tances to which the earth must be thrown. In ordinary soils, one man with a pick can loosen, in a given time, as much earth as two men can remove with shovels, during that time. The amount of eartli removed can be determined experimentally, by having a man to loosen the earth over a given space to the depth of one blow of the pick, and note the time employed. Then have a man to shovel this loosened earth off, and throw it a horizontal distance of twelve feet, and note the time taken to do it. With the data thus obtained, the proportion of picks to shovels can be determined. It is assumed that a workman while shovelling can pitch a shovelful of earth to a distance of twelve feet in a horizontal direction or of six feet in a vertical one. The shovellers in the beginning of the excavation can therefore be placed twelve feet apart ; but as the ditch deepens, the distance apart must be lessened, and finally, the number of shovellers must be in- creased. There should be for every two or three working parties, one man, at least, to spread or level the earth as it is thrown upon the embankment, and another man to ram it. From the foregoing, knowing the kind of soil and 78 FIELD PORTtPICATIOm. the dimensions of the ditch and parapet, the size of each working party cau be estimated. In the field, the workmen are soldiers, taken from infantry regiments, detailed for this fatigue duty The details are divided into squads, forming working parties under the supervision of their non-commis- sioned officers. Several of these squads form a fa- tigue detail under the charge of a commissioned offi- cer and are by him, divided into reliefs according to the circumstances of the case. The soldiers are dis- tributed, as just described, along the line of the ditch, excavating the earth, throwing it upon the embankment, spreading it, and ramming it. There should be present with every fatigue de- tail an engineer soldier, who should explain to the men their duties, before they begin to work ; should advise them how to use their tools ; and should be responsible that the proper slopes and dimensions of the profiles are observed. 74. Time required. — The day's task for an or- dinary workman in common soil is about eight cubic yards, when the earth is not thrown higher than six feet. Half of this, or about four cubic yards, may be taken as a fair day's work for the ordinary soldier on fatigue duty, if he works faithfully. The time for throwing up a work, whose profile is that shown on page 17, may be roughly calculated, as follows: The area of the section of the parapet is about CONSTRUCTTON OF Ft ELD WORKS. 79 one hundred and eighteen square feet. The area laid off for a working party has a length of six feet. It may be roughly estimated that the working party assigned to one of these areas is required to make an embankment containing seven hundred and eight cubic feet, or about twenty-six cubic yards. An ex- cavation which measures twenty- three cubic yards will give earth enough to make a parapet containing twenty-six cubic yards. Calling the amount of exca- vation to be twenty- four cubic yards, and supposing each party to take out only four cubic yards per day, it will require six days to construct the parapet. The parapet, of the dimensions given, can be con- structed by fatigue details, in one-third of this time, or less, by increasing the number of men in each work- ing party, dividing the party into reliefs, and work- ing continuously until the parapet is finished. When the embankment has reached the height of the banquette tread, additional working parties are organized, for the purpose of constructing the revet- ments, which are required to support and protect the interior slope. When no engineer soldiers can be spared to oversee the work, substitutes must be obtained. There will be found, in all of the infantry regiments of the United States service, non-commissioned officers who have had experience in the labors of excavating and embanking. These can be detailed to act as engineer soldiers and 80 MELD FORTIFICATIONS, can fit themselves in a very short time to discharge the duties assigned to them. The method of posting the men is a matter of detail, acquired from practice ; the only condition imposed is that there shall be no crowding. Only a limited num- ber of men can work at the same time, and rapidity of execution can only be obtained by frequent reliefs. Rapidity of execution is facilitated by good judgment, especially in the selection of the men. Good axemen should not be used to dig ; and good diggers should not be used as axemen. Proportioning the work accord- ing to the skill displayed by the men will materially shorten the time required for its execution. CHAPTER IX. REVETMENTS. 75. Definition. — Any facing used to protect a slope of earth from the action of the weather is termed a revetment. (Art. 24.) Embankments of earth are frequently made with slopes greater than the one which the earth naturally assumes. These slopes, when left exposed to the action of the weather, soon wear away and lessen until they coincide with the natural one. Where it is necessary to preserve them, it is done by using revetments, or retaining walls. The latter are necessary when there is a horizontal thrust due to a mass of earth, which would fall were it not supported. The former are used where there is little or no thrust, and the object is principally to protect the slopes from the weather. 76. Materials used for revetments. — If there is time, and the work is to be kept in good con- dition, it is a good policy to protect all the slopes, by using revetments, or by sowing the slopes with grass seed. The interior slope is, however, the only one in a field work that requires protection as soon as constructed. All other slopes can wait until the parapet is com- 82 PIELV FORTIFICATIONS. pleted. Field works are frequently constructed hur- riedly, and, in consequence, the revetment of the in- terior slope must be made quickly. This necessitates the use of any materials suitable for the purpose, and which can be quickly procured. Some of the materials near at hand, or which can be quickly procured, from which the revetments can be made, are fascines, gabions, hurdles, timber, plank, casks and barrels, sandbags, grass sods, etc. 77. Fascines. — A fascine is a long, cylindrical, bundle of brush-wood, bound closely together near the ends and at intermediate points by withes or by wire (Fig. 22). ng.22. A fascine is usually made nine inches in diameter, and eighteen feet long. The method of making them is as follows : Trestles are placed in line, with the end ones sixteen feet apart from each other. If Qve trestles are used, the trestles would be four feet apart. Each trestle is made by driving two stakes, each six and a half feet long, and three inches in RSVETMENTS. 83 diameter, obliquely into the ground so that they will nearly make a right angle with each other, and bind- ing them together at the middle. The brushwood, stripped of its loaves and small twigs, straight and flexible, and about an inch in diameter, is laid in the trestles, as nearly uniformly in thickness as possible, until the trestles are nearly full. This brushwood is compressed into a p,--^ 23, compact bundle by means of a fascine choker (Fig. 23), and bound firmly by bands. The fascine choker is formed of two stout levers, which are connected by a piece of chain or stout rope, as shown in the fig- ure. Its action is easily understood. The bands are usually withes, sometimes called gads, pre- pared for the purpose. The bands are twelve in number, placed at equal distances apart ; those near the ends being six inches from the end of the fas- cine. "Wire, and even spun yarn, are used for the bands at times. Every third or fourth band should be made with a projecting end, terminating in a loop. This loop is used to go around a small picket driven into the earth, and ht)ld the fascine in its place. After practice, a fascine can be made by a squad of five men in about an hour. 78. Fascine revetments. — The revetment of S4 PIELD FORTIFICATIONS. the interior slope, by using fascines, is shown in Fig. 24. Fig. 24. The first row of fascines is partly imbedded in the earth, so that about half of the thickness of a fascine is below the banquette tread. This row is held firmly in place by small pickets driven through the fascine and into the earth on which it rests. The other rows are laid on top of the first row, and on each other, and held in place by pickets ; the fascines in each row breaking joints with those in the row beneath, and are so placed as to have a slope of f. The projecting ends of the bands are stretched, and small pickets are also driven through the loops. These pickets are called anchoring pick- ets. Six rows of fascines would place the top of the upper row within four inches of the interior crest. A row of sods should be laid along this top row to complete the interior slope. If five rows only are used, the portion of the in- terior slope between the top row and the interior crest is left to stand at the natural slope of the earth until REVETMENTS. 85 there is time to finish it. It is usually finished by means of sods. 79. Gabions. — Gabions are basket-work cylin- ders, open at both ends (Fig. 25). The bas- Fig. 25. ket-work is made of brushwood, although iron has been used. A gabion is usually made two feet and nine inches high, and two feet in diameter. The method used ordinarily in making a gabion is to drive pickets, three and a half feet long, into the ground in a circular row. The size of the circle may be deter- °o <> mined by a circle traced on the ground, or by a circular piece of wood called a gabion form. The gabion form is then raised about one foot above the ground, and the pickets are bound firmly together by a rope passed around them just below the form. The brush wood, stripped of leaves and twigs, straight and flexible, and about one-half inch in di- ameter, is then woven around the pickets. The most approved method of weaving is to use three rods as shown in Fig. 25. The three rods are placed with the large ends inwards, and separated from eacli other by an intervening picket. The first rod (the one in rear) is passed over the other two rods, around two pickets, and within one ; the second rod (now the rear one) is passed over the rods, around two pickets, and within one ; the third is manipulated 86 FIELD FORTIFICATIONS. in the same manner. Each rod comes to the front in turn, and is separated from the others by a picket. As the weaving progresses, the basket-work must be kept pressed down, either by the foot, or by blows from a mallet or a stout stick. When the basket- work has reached the height of two feet and nine inches, the* ends of the rods are firmly secured by withes. If the basket work is not of the requisite height, by reason of the pickets being driven too far into the ground, it is turned over and added to on the other end, both ends being secured by withes. The usual number of pickets is nine, but a larger number may be used when the brushwood is flexible and small. With a form, two men can make a gabion in one hour and a half ; without it, it will take three men about two hours to make one gabion. Gabion revetments are rarely used for the interior slopes of ordinary field works. They are much and usefully employed in the trenches in siege operations, in batteries, and in embrasures. When used as a revetment for the interior slope, they give a height of three feet, in consequence of the projecting ends of the pickets. They are made to rest upon a row of fascines half buried in the ban- quette, and are so placed as to have the same inclina- tion as the interior slope. The gabions are then filled with earth, and the parapet is raised behind them. REVETMENTS. 87 When the parapet reaches the height of the gabion, a row of fascines is laid on top of the gabions to give the requisite lieiirht to tlio interior crest. 80. Hurdle revetments. — Hurdle revetments are frequently used in field works. The hurdle is ordinarily two feet and nine inches high, and about six feet long. It differs in construc- tion from the gabion, only in having the pickets placed in a straight line, or along a line which is slightly curved. When used for revetting long lines of parapet, the hurdles are made continuous. The pickets are driven into the ground as close to the parapet as possible, leav- ing only space enough to allow the brushwood being woven in and out and around the pickets. The pick- ets should have the same inclination as that of the in- terior slope, and be tied back by anchoring pickets. A larger size of brushwood can be used for this revet- ment than can be used for making gabions. 81. Timber Revetments. — Fascines, gabions, and hurdles are used for revetments when timber is scarce, or when there are better uses to which the tim- ber may be api)lied. Timber is a suitable material, both on account of its comparative durability, and for the ease with which it can be worked into tiie shapes required. Its scarc- ity, and its usefulness for other purposes, are the reasons for not employing it more freely for i-evet- 88 FIELD FORTIFICATIONS. ments. When abundant, it will be used in preference to any of the other materials for the revetment of in- terior slopes and scarps. There are two general methods employed in mak- ing timber revetments. One method is to place the logs in a horizontal position, piling them in rows one above the other with the proper inclination, ana fastening them in place, by pinning tlie layers to- gether, and by anchoring tics. The other method is to cut the sticks into short lengths and to place them in contact with each other in an upright position , then cap the posts by a horizontal piece of timber. (Fig. 26.) Fig. 26 The latter method is considered to be the better one, especially in the event of a cannon shot pene- trating the parapet and striking the back of the revetment. If the log struck is knocked out, the sphere of injury would be more extended when the log was in a horizontal position than if it were in an upright one. This method of revetting the slopes (Pig. 26) was REVETMENTS. 89 much used in the late war in the United States. General Barnard of the United States Engineers de- scribes it as follows: "This (the revetment) consisted of posts from foui to six inches in diameter of oak, chesnut, or cedar, cut into lengths of b\ feet and set with a slope of \ in close contact in a trench, at the foot of the breast height, two feet in depth. These were sawed off sixteen inches below the crest and shaped to re- ceive a horizontal capping piece of six-inch timber, hewed or sawed, to a half-round, as shown in the sketch." The lower ends of the posts rested upon a two- inch plank, placed in the bottom of the trench. The capping piece was '* anchored '* in place, by ties notched into logs buried in the parapet. The slope of -f- is steeper than that assumed for the ordinary profile. 82. Plank Revetments. — Plank is an excellenl material for revetments where durability and great strength are not required. The ease of working and convenience of handling are its great advantages. When ^'/ can be easily obtained and can be spared for the purpose, it will always be used in works of hurried construction. Revetments may be made with it by driving posts or pieces of scantling into the earth, three or four feet apart, giving to them the same inclination as the 90 FIELD FORTIFICATIONS. interior slope. Boards, in a horizontal position, to re- tain tlie earth, are then nailed to these scantlings or posts. Or, the scantlings may be capped, and the boards having been cut into suitable lengths, placed in an upright position, similar to the posts in the timber revetment, shown in Fig. 26. The moisture of the earth soon produces rot in the boards and renders the revetment a very perishable one. 83. Casks, barrels, etc. — Casks, barrels, and materials of this description, are frequently found in numbers within the neighborhood of the work, and can be usefully employed in making temporary revet- ments. The construction of revetments of this kind will be similar to that in which gabions are used. 84. Sand-bags. — Sand- bags are sacks made of strong canvas, and then filled partially with sand or earth. . They are used quite frequently, when the other materials for revetments can not be easily procured ; or where, from peculiar circumstances, they can be usefully employed to give speedy protection against an enemy's fire. The sacks are usually made about two feet and three inches long, and one foot and two inches wide, when empty. They are about three-fourths filled with earth, and are then tied at the top. The bags are not entirely filled with earth, so that they may be flattened when laid in the revetment. They are usually laid as "headers and stretchers," that REVETMENTS. 91 is, the ends and sides of the bags appear alternately in each course. (Fig. 27.) The bags break joints in each course. Sometimes the bags are all laid as headers. Fig. 27. It will require eighteen sand-bags of the size just given, laid as headers and stretchers, to make one square yard of revetment. The sand-bag revetment possesses peculiar advan- tages, where speedy shelter is required, or where the earth has to be carried some distance under fire. Its great defect is the perishable nature of the revetment, due to the speedy rotting of the bags. Fig. 28. Sand-bags are very useful to place on the parapet of a work to form improvised loop-holes for tlic de- fence. (Fig. 28.) 92 FIELD FORTIFICATIONS. 85. Sod revetments. — Grass sods form the best revetments for the interior slopes and for the cheeks of embrasures. Sod revetments are durable and do not splinter when struck by the enemy's projectiles. They require considerable time in which to construct them. Sods for revetments should be cut from fine, close turf with thickly matted roots, the grass having been mowed previous to the cutting. They are cut in the form of blocks, 16 inches long, 8 inches wide, and 4 inches thick. The construction of a sod revetment should be com- menced, when practicable, as soon as tlie parapet is raised to the level of the tread of the banquette. The first course is then laid with the grass side downwards, alternately as headers and stretchers ; or, two stretchers to one header. The upper surface of this layer should be perpendicular to the plane of the interior slope, and this position is given to the sods by sloping the bed on which they rest. The second course is then laid in a similar manner on the first, and breaking joints with it. Sometimes a few wooden pegs are driven through the upper sods into the lower ones, to fasten them together; but this is not necessary. The other courses are laid in a similar manner to that described for the first and second, until the top RE VETMENTS. 93 course is reached ; which should be Jaid with the grass side upwards. Each course is settled firmly iri place by tapping the sods as they are laid, with the flat side of a spade, and packing the earth of the parapet closely beliind each layer. It is recommended to have the sods laid protruding slightly beyond the face of the interior slope, so that each course, after it has been laid, may be nicely trimmed. In the sod revetments used in the Washington de- fences, the sods were eighteen inches long, twelve inches wide, and four inches thick. They were laid to form a sod-wall, twelve inches thick, and with a slope of f. The earth of the parapet was thoroughly rammed behind the revetment, and was carried up simultane- ously with the laying of the sods. The courses were connected by small wooden pegs, three-fourths of an inch in diameter and nine inches long, driven through each alternate course into the layers beneath. 86. Pisa Revetment. — A pisa revetment is a wall of clay built against a slope to protect it. Its construction is as follows : Common earth, mixed with clay and moistened with water, is kneaded until the particles will adhere when pressed or squeezed together. Sometimes chopped straw is mixed in the mass. A row of pickets, with the proper inclination given to them, is driven along the foot of the interior 94 FIELD FORTIFICATIONS. slope, the tops extending a short distance above the height marked for the interior crest. A shallow trench about twelve inches wide is dug in the para- pet, behind the line of pickets, and a board laid hori- zontally on edge on the side next to and supported by the pickets. The tempered clay is then placed in the trench and rammed. Successive layers are placed in until the clay reaches the top of the board, the earth of the parapet being carried up simultaneously with the revetment. A second board is then placed upon the first, and the clay rammed in, rising si- multaneously with the parapet as in the first course ; and this process is continued until the top layer is on the same level with the interior crest. When the clay has dried, the boards and pickets are re- moved. 87. Other revetments. — Sun-dried bricks, adobes, and stones, have been used for revetments. They are built into walls in a method similar to that described for sods. Any material which can be used to protect the earthern slope will be suitable. Time, durability, and fitness are the things to govern in the selection of materials for revetments. For batteries or parapets exposed to artillery fire, the fascines are preferable for revetments. Gabions are to be preferred for embrasures and traverses ; sand bags for powder magazines and traverses. Some- REVETMENTS, 95 times all three of them are used simultaneously in the same field work. Gabions require less wood than fascines, and are more easily made. They do not have to be tied or fastened in place, like fascines. Sand-bag revetments are more quickly made, but the canvas rots quickly. Sods are the best material, but revetments made from them require a great deal of time and labor to construct. CHAPTER X DEFILADE. 88. Defilade. — The first principle, enunciated m A.rt. 7, imposes the condition that the defenders of a field work should be screened from the enemy's view. If the work is built upon a level site whicli com- mands all the ground around it, this condition is ful- filled, when the space occupied by the men is enclosed by a parapet eight feet high. If the work is built upon an irregular site, or if there are heights within cannon range which com- mand it, a parapet of uniform height and only eight feet high will not screen the men in the interior from the enemy's view. The arranging of a work, under these circum- stances, so that the men standing on the terreplein behind the parapet shall be screened from the enemy's view, and be protected from his fire, is called defilading the work. There are three ways which may be used to defil- ade a work. 1. By raising the parapet. 2. By lowering the terreplein. 3. By using traverses. DEFILADE. 97 Tlie labor of defilading may be reduced, and in many cases avoided, by giving proper directions to the lines of the work. 89. Plane of defilade. — To illustrate this sub- ject, take a field work built upon a site which is l)i"iictically level, but within cannon range of a hill which commands it. (Fig. 29.) Fig. 29 It is plain that if the parapet from C to D is not high enough, an enemy occupying a position at M can see the defenders when standing on the terreplein be- hind the parapets of the flanks, A C ^^^d B D. It is also plain that if the parapet be raised high enough to conceal the men on the terreplein at A and at B> an enemy at M can not see the defenders on the terreplein in any portion of the work. 98 FIELD FORTIFICATIONS, Safety for a man at A or B requires that the line of fire coming from M should pass at least six feet and six inches above the ground on which he stands (Art. 20). In other words, he must be sheltered from all fire of the enemy coming from |V|, Let H P represent a line which passes eight feet above the ground along A B> and five feet above the ground at M • this will be a line of the plane of fire from M, above which the effects of direct fire from IVI may be neglected. Any mass interposed be- tween the line A B and the height M,- and raised until its top touches or rises above the line H P^ wiL intercept the direct fire on A B, from M, and shelter the men on the terreplein at the points A and B, If then the interior crests S C and S D are held in a plane passing eight feet above the ground at A B- aud five feet above the ground at M, the parapets of these faces, S C and S D? will intercept this fire, and the work will be defiladed. This plane in which the crests S C and S D are held is known as the plane of defilade, and may be defined to be that plane, which containing the in- terior- crest of a tvoj^k, passes at least eight feet above those points to he sheltered, and at least five feet above the ground luhich can he occupied hy a7i enemy luithin cannon range. 90. Area to be defiladed.— The amount of space in rear of a parapet which is required to be de- DEFILADE, 9& filaded, depends upon circumstances. In some cases, the entire space enclosed, and in others only a part, is to be protected from this fire from a commanding height. Thus, it is usual to require that the whole interior space of an enclosed work should be defiladed ; that the interior as far as the gorge should be defil- aded for a half-enclosed work ; and that so much of the interior, or so much of the terreplein behind the parapet, as may be necessary for the free movements of the defence, should be defiladed in open works or lines. 91. Method of determining the plane of defilade. — It is not convenient in practice to place the eye at a distance of eight feet from the ground, nor is it an easy thing to judge, from a distance, what should be the position of a point like P, which shalL be five feet above the ground. The method used is to place the ej^e at a convenient distance from the ground, observe the highest point of the top of the hill, and determine the position of a visual plane tan- gent to the hill. Knowing the position of this visual plane, a second plane is passed parallel to it and five feet above it. The tangent visual plane is known as the rampant plane, and the i)lane parallel to it is the plane of defilade. The position of the rampant plane and the plane of defilade may be fix^ed as follows. To illustrate the mctliod, a redan is supposed to be 100 FIELD FORTIFICATIONS, the field work which is to be built, upon a position commanded by a neighboring height, and that the salient and extremities of the faces are marked by upright poles planted in the ground. (Fig. 30.) Tlie trace marked,, the next step is to profile the work, and this requires the height of the interior crest to be de- termined. Two stakes, at a convenient distance apart, are erected upon the gorge line ; if not too far apart, the poles already erected to mark the extremities of the faces may be used. A line, three feet from the oTound, is marked by a strip of wood having a straight edge, or by a cord tightly stretched, and fastened to these uprights. An observer is placed in rear of this line ; he sights along it and tangent to the hill, 7i, and determines where the visual plane containing this line cuts the pole placed at the salient. This point is marked, and DEFILADE. 101 with the line joiniug the two upriglits on the gorge line, fixes the position of a plane tangent to the hill and three feet above the ground at the gorge. If on the three posts, 1, 2, 3, points be marked, five feet above the points of intersection of the posts by the rampant plane, these will be points of a plane which will pass eight feet above the ground at the gorge and five above the ground at h. If the faces of the redan are held in this plane, the whole in- terior of the redan will be defiladed from this hill, and the last plane determined will be the plane of defilade. The extremities of the faces at the gorge have parapets of the ordinary height, viz., eight feet; the parapets from these points, increase in height until the salient is reached, where the height is the greatest. The height of the interior crest can then be deter- mined, at the points where the profiles are to be placed. In the previous chapter, the height of the parapet is supposed to be uniform throughout. The site being level, there is no reason why any one part of the interior crest should be higher than another. It is nevertheless the practice, even in this case, to give ad- ditional height to the parapet at a salient, not for the purposes of defilading the interior, as just explained, but to lessen the effect of any enfilading fire which an enemy might obtain upon the faces, and to allow for 102 ' * ' ' i>f^li) PoiiTtFtcATtom. the descent of the trajectory of a projectile which might graze the interior- crest at the salient. 92. A slight deviation from the method just des- cribed is made when the work to be defiladed is a lu- nette, instead of a redan (Fig. 29). Two uprights, about twelve feet apart, are planted upon, and near the centre of, the gorge line. A third upright is placed in front of the gorge and ten or twelve feet from it, upon the line joining the centre of the gorge line with the salient. The points are then marked where the rampant plane, three feet above the ground, cuts these three uprights and the uprights planted at the salient, S? and at the shoulders, C and D- A distance of five feet is marked above the points just determined, and this will fix the position of the plane of defilade for the lunette. 93. Front and reverse defilade. — The sup- position has been made that there is only one com- manding height within cannon range of the field work. Suppose that there are two or more heights, from each of which an enemy could see the interior of the work, if an ordinary uniform height of parapet is used. Take the case of a redan. There are two distinct conditions which may arise. One of them is where all the heights will lie within the angle of the faces prolonged ; the other is when one or more of these heights will lie without this angle. Thus, the redan, DEFtLADia, 103 A S B, (Fig- 31) has the heights Z ii^d Z', lying within the angle of the faces prolonged ; and the heights, X ^"^ Y> lying without tliis angle. Fig. 31. The heights Z and Z' can only bring front fires, direct or slant, upon the faces of the redan. The height X can bring a direct fire upon the face A S? and a reverse fire upon the face B S- A similar condition exists for the height Yi The arrangements made to screen the terrepleins from the front fire are all made like those just de- scribed, and give rise to the problem known as front or direct defilade. Those made to screen the ter- replein and the men on the banquettes from reverse fires give rise to the problem of reverse defilade. The method used to defilade a work from two or more heights, when these heights can only bring front 104 FIELD FORTIFICATIONS. fireS; is the same in principle as thafc used for a single height. It is only necessary in a case of this kind to defilade the work from that height which gives the highest fire. When some of the heights expose the work to a reverse fire, it is usual to defilade the faces separately from the front fire, and then make the necessary ar- rangements to protect the interior against reverse fire. 94. Method used to defilade a work ex- posed to reverse fire. — The first step, as has just been stated, is to defilade the work from the front fire. It is evident, even when this is done, that men upon the banquette are exposed to reverse fire. Thus, suppose a redan like that in Fig. 31, to have had its faces defiladed from front fire. It is plain that men, standing on the banquette at B? would be above the plane of defilade containing the face A S? and would be seen by an enemy at X* Suppose the interior crest of the face, A S? to be raised high enough to shelter the men on the banquette of the opposite face, Fig. 32. B S- The effect would be to shelter the men there, but to expose the men on the banquette of the face A S. Thus, if the crest at C> (Fig. 32), is raised to DEFILADE. 105 shelter the men oq the bjinquette at D, it exposes the crest at F to a fire coming from £. The defect of re- verse fire is not removed, but only transferred to an- other place by raising the parapet. This method can not be used. The method that is used is to interpose an auxiliary mass which shall intercept the dangerous reverse fire. A mass of earth, placed across a line of fire to in- tercept the missiles moving along that line, is termed a traverse. Thus, if a traverse of earth is placed between the banquettes and made thick enough to protect the men, and high enough to intercept the enemy's missiles, the men can stand on the banquettes without fear of the reverse fire. The thickness of a traverse, like that of the para- pet, depends upon the penetration of the enemy's pro- jectiles. The height of the traverse depends upon the height of that plane of reverse fire, above which the fire may be neglected. This plane which determines the height of the traverse is known as the plane of re- verse defilade. It may be defined to be, that plane which passes at least tiuo feet above the interior crest exposed to reverse fire, and at Uast five feet above the ground from which this fire caii come. When both faces of the work are exposed to re- verse fire, there will be two planes of reverse defilade. In this case, the least height of the traverse will be 106 FIELD FORTlFICATIOm. determined by the intersection of these planes of de- filade, and the line of intersection will form the mid- dle line of the top of the traverse. If it be more convenient to place the traverse along some other line, the capital for instance, the height of the tra- verse will be determined by the higher of the two planes. A traverse placed along the capital frequently crowds the salient too much ; in which case, its direc- tion is changed when near the salient, and it is joined to the face most exposed. (Fig. 31.) 95. Method of determining the plane of reverse defilade. — Since a traverse is to be used, it will be supposed that its place is along the capital. This traverse will divide the interior space into two equal parts. It will only be necessary to defilade each half from the front fire ; which may be done as fol- lows : Uprights are placed along the capital to mark the middle of the traverse. A line, three feet from the ground, is marked upon these uprights by stretching a cord, or by nailing strips of wood to the poles. The rampant planes passing through this line, and tangent to the hills on each side which furnish the highest front fire, are then determined, and their inter- sections with the uprights placed along the sub-crests of the faces, are marked. On these verticals the planes of front defilade are then marked, and these DEFILADE. 107 points, thus obtained, determine the height of the in- terior crests of the faces. The profiles are then placed along the sub-crest at the proper points. T)ie next step is to determine the position of the planes of reverse defilade. To illustrate the method, it will be shown how I he plane of reverse defilade is found for the face, S B» of the redan in Fig. 31, which is exposed to a reverse fire from the high ground at X, It is supposed that the faces, S A J^-^d S B? have oeen defiladed from front fire by the method just de- scribed ; that the profiles of the parapet have been erected ; and that the vertical poles, marking the po- sition of the middle line of the traverse, are in place. Two, or more of the profiles along the face S B are taken, and are joined by a straight line which is just three feet below the interior crest of this face, measured on these profiles. The position of this line is fixed usuallv bv markinor on the ver- tical on the sub-crest of each profile, a distance of eighteen inches above the banquette, and by joining the points thus determined by a strip of wood or a tightly stretched cord fastened to the profiles. An observer is placed behind this line and deter- mines the position of a visual plane which contains the line, and is tangent to the hill, Xi The points of in- tersection of this visual plane with the poles placed to indicate the position of the traverse, are marked. lOS fItJLD PORTIFtCATtONS. These points marked on tlie polos, witli the line three feet below the crest, fix the position of the rampant plane for the reverse defilade. The distance of five feet above this plane, is marked upon these poles, and the points marked thus will be points of the plane of reverse defilade, and will determine the height of a traverse required to protect the face B S from the reverse fire from X* A similar process determines the plane of reverse defilade for the face A S- The position of the higher plane will determine the height which is to be given to the traverse. The form of a traverse and its construction will be men- tioned hereafter. 96. Defilade by traverses. — The problem of reverse defilade is almost always solved by the use of traverses. That of front defilade, by either raising the parapet or lowering the terreplein. The latter plan is the method pursued in permanent fortifica- tions ; it is sometimes used in ordinary field works, but rarely, because the terreplein and surface of the ground are generally one and the same. It is not always practicable to solve problems of front defilade in field works by raising the parapet, because the height required for the parapet to reach the plane of defilade may be too great, and exceed the limit laid down. In these cases, traverses are used. As an example, take a lunette coinminded by a \ DEFILADE, 109 hill, as X (^ig- 33) and suppose that, with a com- mand of twelve feet at the salient, the interior crest protects only lialf of the interior of the work. The remaining half can only be protected by means of a traverse, P, placed along the line dividing the part which is defiladed from that which is not. Two short traverses would be placed near the shoulder angles, overlapping the ends of the principal traverse, as shown in the figure, with room between the tra- verses to allow of a passage-way. The short traverses at C ^^i^ D would be used, even if P were not, to protect the men on the ban- quettes of the flanks from slant reverse fires coming from H i^»d K' If this traverse P is placed in the work to shelter the men from u fire in the opposite direction, wliich would be a reverse fire upon the faces, it is termed a parados, a name employed to designate tlie traverse wJien it k used to intercept a reverse fire of this kind. The great objections to the use of traverses are the space they take, and the labor they require for their construction. When built they are often utilized in no FIELD FORTIFICATIONS, other ways, besides the primary use as an intercept- ing mass. 97. Defilade of lines. — The same methods may be used to defilade an open work, a part of a line, or a line, as are used to defilade a half-closed or a closed work. The longest sides, or the faces, of an open work or a line should be traced, where practicable, perpen- dicular to the direction of the enemy's fire ; and should be locaceu, if possible, so that the parapet shall occupy a ridge, or be so situated that the ground in rear of the parapet shall slope away from it. A line traced and placed in this way will, for the same height of parapet, defilade a larger extent of terreplein than it would if its direction were inclined to that of the fire, or the parapet were placed upon ground which was level, or which sloped towards the enemy. When the commanding ground is a ridge or suc- cessions of hills on the same general level, the direc- tion of the line should be parallel to the general direc- tion of the ridge. (Fig. 34). DEFILADE. Ill The simple right line with a uniform relief will, iu this case, defilade the terreplein more efficiently than if the line were broken, since the general direction of the line is peri)cndicular to the general* fire coming from the heights. If, however, a broken line is to be used, a contin- ued redan line for example, it is recommended to make the salients of the large redans as obtuse as pos- sible. By so doing, the perpendicular distance be- tween the salients and re-entrants is shortened, and the long faces are made more nearly perpendicular to the enemy's close fire. If the ridge is not too high, and the distance from the salient to the re-entering is not too great, it will be practicable to hold the interior crest of the line in a plane of defilade, and not have the height of the parapet too great at the salients. Moreover, the ob- tuseness of the salients will cause the prolongations of the faces to intersect the high ground at distances so great, that the enfilade fire from these points may be disregarded. If the heights are not on the same general level but slope downwards to the plane of site, a better position for the line is one which is oblique to the crest of the ridge. A direction should be given to the line, so that its front, pro- longed, would pass through the point in which the meridian line, or vertical contour, of the ridge jnorcos the plane of site. Thus, a ridge of high ground, 112 FIELD FOR TIFICA TIONS. sloping downwards from X ^o Y (Fig. 35), commands the ground, A B. on which a line of fortification is to be built. Fig. 35 The vertical contour line of the ridge is the intersec- tion of a vertical meridian plane with the ridge, which line is projected upon the plane of site in the dotted line X Y, ^^ii^ pierces this plane, in this case, at B, Th6 front of tlie line should have a direction like A B. In this position a plane can be passed in whicli the salients, with a uniform command, may be held, and the terrepleins be defiladed from the high ground in front. A straight line of parapet would, in this case, afford the best cover for the terreplein behind the DEFILADE, 113 parapet, as it would in the case where the line is taken parallel to the commanding ground. But, if a broken line be used, care should be taken, as in the last case, to make the salients obtuse angles. If the line be a bastioned line, it is plain that the flanks are liable to be exposed to both reverse and enfilading fires. The nearer the angle A B X is to a right angle, the better will be the position of the line A B to allow of its terreplein being defiladed from the high ground in front. U' CHAPTER XL THE INTERIOR ARRANGEMENTS OF A FIELD WORK. 98. Classes.— The earth work for the parapet being completed, and tlTe revetments of the interior slope constructed, attention is then paid to the interior of the work. Certain arrangements have to be made in the interior, to add to the efficiency of the defence, and to provide for the comfort of the troops who have to occupy the work. These interior arrangements are divided into classes, according to the object to be attained by them. The divisions may be classified as follows : 1. The arrangements of, and along a parapet, in- tended to add to the efficiency of the defence ; 2. The arrangements within the area enclosed by a parapet, to shelter the men and materiel from the fire of the enemy : 3. The arangements made to allow egress and ingress of the troops ; including those made to guard the outlets against surprise ; and 4. The arrangements which may be made to provide for the comfort and welfare of the garrison when occu- pying the interior of the work for some time. INTERIOR ARRANGEMENTS OF A FIELD WORK. 115 I. Arrangement of the Parapet. 99. Defence. — The work may be defended by musketry alone, or it may be defended by artillery combined with musketry. The arrangements of the parapet for musketry are completed when the banquette and the revetment of the interior slope are finished. The work, in this condition, ^oes not admit of the use of artillery. Some additional arrangements must be provided, if artillery is to be employed. The fire of artillery is either over the parapet or through it. In the former case, the fire is said to be " en harhette; " in the latter, it is called *' embrasure fireJ^ In both of these fires, arrangements must be made in or along the parapet for the service of the guns. 100. Barbette fire. — Barbette fire can only be obtained by some arrangement which raises the gun into a position from which it can be fired over the parapet. There are two methods by which this can be done ; one, by mounting the gun on a high carriage, or on a carriage which admits of the gun being raised to the necessary position ; the other^ by building a mound of earth sufficiently high behind the parapet, and placing the gun on this mound. The latter is the method generally employed in field works. This mound of earth in rear of the parapet, with its upper surface arranged so that a piece of artillery 116 FIELD FORTIFICATIONS. placed upon it can fire over the parapet, is called a barbette. The artillery used, in the defence of field works, may be either siege or field guns ; but most gener. ally the latter are employed. The upper surface of the platform on which the wheels rest, or the upper surface of the barbette, when no platform is used, should be at a distance below the interior crest just sufiicient to allow the gun to fire over the interior crest and parallel to the superior slope. A distance greater than this would interfere with the efficiency of the gun ; a distance less, would unneces- sarily expose the gun and carriage to the enemy's fire. The axis of the trunnions of a field gun, in the United States service, is about forty-three (43) inclies above the ground on which the wheels rest. The diameter of the piece at the muzzle and the inclination of the superior slope being given, it is easy to deter- mine what should be the distance of the upper sur- face of the barbette below the interior crest. The general rule used is to take this distance at two feet and nine inches for field guns, and four feet for siege guns, when mounted upon the ordinary car- riages. 101. Construction of the trace of a bar- bette. — The trace of a barbette for a single field gun behind the parapet of one of the faces of a work, may be constructed as follows : INTERIOR AKRANOKMEWTS OF A FIELD WORK. 11? Let A B (Fig. 3G) be the interior crest of the face, and C the point at which the barbette is to be constructed. Fig. 36. Superior Slope Banquette The upper surface of the barbette is to be made level, and large enough to allow the working of the gun. A field gun firing over a straight parapet requires a front of ten feet, at least, and should have a depth of twenty feet, to allow for the recoil and proper handling of the piece. Since the upper surface of the barbette is thirty-three inches below the interior crest, its intersection with the interior slope will be a straight line, a b, parallel to A B> and eleven inches 118 FIELD FORTIFICATIONS, from it in projection. Make a h equal to the re- quired length of front (taken to be fifteen feet in this case) and through the extremities, draw the lines a c and h d perpendicular to a h, making them each equal to twenty feet. The area abed will be the upper surface of the barbette, in plan. The uppei surface of the barbette is joined to* the site by the natural slope of the earth, which is assumed to be {. The height of the parapet is eight feet, which fixes the height of the barbette at five feet and three inches. The intersections of the natural slopes of the sides and rear with the plane of site are then easily determined, as well as the intersections with the ban- quette tread and the banquette slope. These intersec tions determined, the construction of the trace is com- pleted. The gun is carried to the top of the barbette bj an inclined road, called a ramp, connecting the upper surface of the barbette with the plane of site. The ramp, f g f g\ has a width of nine feet and a slope of -J-, and is here placed at the rear of the barbette. The width, the slope, and the position, are all governed by convenience and the circumstances of each case. The sides of the ramp are joined to the site by the natural slope of the earth. 102. Barbettes, when used, are almost always placed in the salients. Their construction is practically INTERIOR ARRANGEMENTS OF A FIELD WORK. 110 the same as when phiced against the face of a work. It is usual, when a barbette is made in a salient, to fill the salient angle for a short space with earth, and to form a short face, called a pan-coup6, (Fig. 37), Fig. 37. over which the gun fires in the direction of the capi- tal. The length of this pan-coupe is from ten to fifteen feet. The construction of the plan differs from the one described only in tin* form of the upper surface. In this case, the upper surface is pentagonal in form, care being taken to make it large enough to allovv the gun to be fired over the faces of the salient, as well as along the capital 120 FIELD FORTIFICATIONS, In this particular case, it is arranged for only one gun. It muy be arranged for several. 103. Embrasure fire.— The opening made in a parapet to allow a piece of artillery to fire through it, is called an embrasure. The gun may rest upon the natural surface of the ground in rear of the parapet, or it may be placed upon a mound of earth similar in shape to a bar- bette, only not so high. The. bottom of the embrasure, in either case, must be placed so that the gun, resting on its platform can fire through the embrasure when run " in battery." The bottom of an embrasure is called the sole. It is given an inclination, to allow the gun to be fired with its muzzle depressed. This inclination is usually taken to be the same as that of the superior slope of the parapet, unless otherwise stated. The opening, a ^ G H, (Fig. 38), in the interior slope is called the throat, and should be made as small as practicable. It is usually made from eigh- teen to twenty-four inches wide, and either rectangular or trapezoidal in form. The embrasure widens towards the exterior, the amount of widening depending upon the extent of field of fire required. This widening is called the splay. The sides of the embrasure are called the cheeks | the line, M N, which bisects the sole is called the directrix. INTERIOR ARRANGEMENTS OF A FIELD WORK. 12 J The portion of the interior slope which is below the throat of the embrasure is termed the genouill- ere j the mass of parapet between two embrasures is termed a merlon, The embrasures in field works are usually cut after the parapet has been constructed, and, in important works, the exterior openings are masked until the moment to use them arrives, to prevent their position being discovered by reconnoitering parties of the enemy. A detail of six men should be able to cut an embrasure in the parapet of a field work and finish it in eight hours. 104. Trace of an embrasure.— To make the trace of an embrasure, draw the line M Ni (Fig. 38), perpendicular to the interior crest, at the point where the line of fire of the gun intersects it, and set oil a h 122 FIELD FORTIFICATIONS, equal to the width of the throat at the top. If the throat is to be rectangular, draw through a and h the lines a G and I H, perpendicular to the interior crest, and produce them until they intersect the line, G H. drawn parallel to the interior crest and at a dis- tance from it equal to one-third of the depth of the throat. This gives the plan of the throat. To trace the plan of the sole, the inclination and splay must be known. Knowing these, the intersec- tion of the sole with the exterior slope can be deter- mined. This intersection may be determined as fol- lows: Construct a profile of the parapet, and find the point in which the line G H pierces it. Through this point thus determined draw a line in the plane of the profile, making the same angle with the horizontal that the sole makes with the horizontal plane, and find the poinc in which this line intersects the ex- terior slope. This point of intersection is a point of the line E F, which line is parallel to the exterior crest. The splay of the sole is usually determined, in plan, by giving to f p some definite length, and then joining its extremities with the lower line of the throat. A throat twenty inches wide will have a horizontal field of fire of twenty- two degrees, when E F is equal to one half the thickness of the parapet ; a fire of thirty- one degrees, when the f F is equal to two-thirds of the thickness; a fire of forty-eight degrees, when this line is equal to the thickness of the parapet. Supposing twenty degrees to be the field of fire required, E F is made equal to half the thickness of the parapet, and the points E and p are then joined to G and Hj which completes the plan of the sole. The cheeks are determined by setting off on the exterior crest, on the right and the left of the sole, points as C and Q, the horizontal distances of which from the lines E G and f \\, measured on the line C Dj shall be equal to one-third of their hciglit above the sole ; and then joining these points to a and b. and to E and Fi by straight lines. These lines complete the plan of the embrasure. The cheeks are warped surfaces which may be gen- crated by moving a straight line, touching the sides of the throat, and the sides of the opening in the exterior slope, so that the intersection of the surfaces with the sole and the superior slope shall be straight lines. The cheeks should be revetted for several feet at least from the throat, to protect them from the weather and the blast of the guns. Gabions are usually em- ployed for this purpose, and are covered in action with raw-hides when they can be procured. Sods make good revetments for the cheeks, if there is time to finish them. Consecutive embrasures should not be nearer to 124 FIELD FORTIFICATIONS, each other than fifteen feet from centre to centre, to prevent crowding of the guns, and to prevent the mer- lon, M, (Fig. 39) from being too weak. A merlon Fig. 39. which measures less than six feet on the exterior crest should not be allowed, as it would make the parapet too weak. 105. Direct and oblique embrasures.— A direct embrasure is one in which the directrix is per- pendicular to the interior crest at the point of intersec- tion with the crest ; an oblique embrasure is one in which the directrix makes an angle with the interior crest. When possible, direct embrasures are the ones which are made. If oblique embrasures are to be made, their method of construction is practically the same as that given for direct embrasures. Oblique embrasures do not admit of the muzzle of the gun being inserted so far as the direct ones, and they weaken the parapets more. Oblique embrasures are not used, as a rule, if the directrix makes with the normal to the crest an angle exceeding ten degrees. In case the angle is greater, the embrasure is provided for, in field works, by modi- fying the interior crest by means of the method known as 'indenting." INTERIOR ARRANGEMENTS OF A FIELD WORK. 125 This method consists in making the crest a cre- maillere line, instead of a right line, with the short branches perpendicular to the direction of the fire, and in these sliort branches constructing direct embrasure?. 106. Comparative advantages and defects of the two kinds of fire. — The advantages of barbette fire in a field work consist in its wide field of fire, its commanding position, and its not weakening the parapet to obtain this fire. The main defect is the exposure of the men and guns to the enemy's tire. The advantages of embrasure fire over the barbette are, that the men serving the guns, and the guns themselves, are both protected to a greater degree from the enemy's fire. The defects are, its limited field of fire, its weakening the parapet to obtain this fire, the constant repairing of the cheeks of the embrasures when in use, and the good mark the embrasures offei to the enemy. 107. Bonnettes. — It is frequently desirable that the height of the parapet, at certain points, should be increased for a short distance. This increase is generally obtained by making use of the constructions known as bonnettes. A bonnette extends but a short distance along the parapet, is made of earth, and is used generally to give greater protection to the men standing on the banquette against a slant or an enfilading fire of the enemy. 126 FIELD FORTiFlCAtlO^S. Bonneties ure placed usually on the salients ; they are sometimes placed on the parapet between guns They may be constructed during the progress of the work, or after the work has been finished. In the former case, their construction is, to all intents and purposes, similar to that of the parapet. In the latter case, they are constructed generally in haste, and sand bags or gabions filled with earth are used to build them. 108, Loop-holes. — Troops on the banquette, when in the act of firing their pieces^ are frequently exposed to the fire of the enemy's sharp-shooters. Under these circumstances, expedients must be devised to protect the men, without interfering with their fire. The expedient which is most generally used, is that of an improvised loop-hole. The loop-hole is made, in this case, by arranging two or more rows of sand bags, placed upon the parapet and filled with earth, so that the top row will be higher than the men's heads, and so as to leave intervals between the bags in the lower rows, through which the men can aim and fire their pieces. (Fig. 28.) Gabions are also used for a similar purpose. The gabions are placed in pairs upon the parapet and filled with earth, each pair being separated from the adjacent pair by an interval of about two inches. A contrivance adopted in the war of 1861-5, was TNTERTOR AHRAXGEMENTS Of A FIELD WORK, l^t quite effective for the same purpose. Skids were placed upon the parapet, with notches cut in them. A heavy log was placed on the skids, occupying a po- sition parallel to the interior crest and just in contact with the superior slope. Notches were cut in the under side of this horizontal log and these were used as loop-holes. The openings to the exterior were made as small as possible, and in some cases were protected by small plates of boiler iron spiked upon the log. When exposed to artillery fire, earth was banked against the log. A wooden loop-hole was devised by Lieut. King (now Major) of the United States Engineers, which was used in 18G4. It was practically a wooden hopper made of boards, placed upon the superior slope of the parapet, and covered with earth. The splay of the sole and the angle of the cheeks were made to suit the field of fire required. The exterior orifice of a loop-hole for musketry should be made as small as possible. A width of tioo inches and a height of five, is sufficiently large for ordinary purposes. The sides are sloped, and an inclination given to the bottom and top, according to the field of fire which is to be swept. Embrasures are sometimes protected in a manner similar to this arrangement for loop-holes. Timbers are laid across the embrasure, covering the throat, leaving only room for the muzzle of the piece. These 128 FIELD FORTIFICATIONS. timbers are then covered by saud bags, by fascines, 3tc., to make them shot-proof. Sometimes the em- brasure is filled in with sand bags or fascines to mask it, these things being quickly removed when the em- brasure is needed for use. Thick wooden shutters, made bullet-proof, and placed on vertical axes, and iron shutters swung on horizontal axes, have both been used to close the throat of the embrasure. In some cases, timber supports were extended back from the parapet and a covering -of timber and earth placed upon them, protecting the gun from vertical and plunging fire. A gun thus sheltered is said, to be case-mated. 109. Traverses. — The traverses constructed along a parapet are of two kinds, viz., the traverses built to afford shelter against slant and enfilading fires, and those built as a protection against the frag- ments of bursting shells. Traverses may be built at the same time that the work is constructed, or they may not be built until there is an immediate necessity for them. In the former case, their construction is in all things similar to that of the parapet, viz., tracing, profiling and execution. In the latter case, they are generally built in great haste, and profiles are not used. The construction is of the simplest kind, hav- ing for its object to interpose a mass of earth upon INTERIOR ARRANGEMENTS OF A FIELD WORK. 1*29 a line of tire, iti the shortest time possible. This is done by piling sand-bags, filled with earth upon the spot to be occupied by the traverse, and raising there a mass thick enough and high enough to serve the end required. Gabions filled with earth are frequently used for the same purpose. The top of a traverse is usually made ridge-shaped, 80 as to carry away the rain water which falls upon it. The sides of the traverse are sloped, the inclination of the slopes being the same, or different, according to the degree of exposure of the traverse to the enemy's fire. The traverse shown in Fig. 40 is an example of a Fig. 40. traverse built to shelter the men on the banquette from a slant or enfilading fire, coming in the direction shown by the arrow. Its top is made ridge-shaped. The side towards the enemy has the natural slope of the earth ; the opposite side is made steeper, and should be revetted. The thickness of the traverse depends upon its exposure to the enemy's fire. If a fire can 130 PtELly FORTIFICATIONS. be brought directly upon it, it should have the same thickness as that given to the parapet. Its height and length depend upon the amount of banquette and terreplein which are to be defiladed by it. The manner in which this traverse is joined to the parapet is shown in Fig. 41, which represents its plan. Fig. 41. The slope on the side toward the enemy is shown, in both of these figures, to be uniform. This is not always the case. The portion exposed to the enemr's fire is given the natural slope of the earth ; but below this plane of fire, the slope may be revetted, and made steeper. Instead of being ridge-shaped, the traverses are, in many cases, made with a cross section similar to that of the parapet. 110. Splinter-proof traverses. — A traverse tNTERtOn ARRANGEMENTS OF A PlELT) WORK. 131 intended to be used only as a protection against splin- ters and the fragments of shells scattered around by tlieir explosion, is known as a splinter-proof tra- verse. Traverses of this kind are not made so thick, nor so high, as the traverses just described. Their usual lieight is the same as that of the parapet. Their thick- ness at the base is from seven to eight feet. Their length varies, being in some cases only ten feet, and in others as much as sixteen feet. As a rule, a traverse of this kind is not joined to the parapet, but is separated from it by a narrow pas- sage which can be used by the men to pass from one side of the traverse to the other. Fig. 42. A splinter-proof traverse (Fig. 42) is usually con- atructed as follows : A rectangular space is marked upon the ground for the base of the traverse. A row of gabions is then 132 FIELD FORTlFlCATlom. placed in juxtaposition along the line representing the base of the traverse, and given a slope inwards, either by setting the gabions on a slightly inclined excava- tion in the ground, or by raising the outer edges by means of fascines laid along on the ground. The gabions are then filled with earth, and also the interior space enclosed by them. When the earth has risen above the top of the gab- ions, two rows of fascines are laid upon the top of the gabions to form a base for a second row of gabions. This second row is then filled with earth, and the pro- cess of filling with earth goes on, until the earth rises high enough. The top is rounded off, or made ridge- shaped, and the traverse is completed. The same method may be used for the construc- tion of traverses required for defilade, when there is a pressing emergency for them. Splinter-proof traverses are placed between the guns along a line of parapet which is exposed only to a direct fire from the enemy, and are only intended to confine the effects of bursting projectiles to a limi- ted space. They are usually constructed only when there is a necessity for them, and then hastily. Gabions, sand bags, fascines, or any of the materials used for revet- ments, may be employed in their construction. 111. Platforms. — A field gun or any piece of artillery, after repeated discharges in the same direc- INTERIOR ARRANGEMEyTS OF A FIELD WORK. 133 tion, soon wears . the ground under its wheels into ruts, if these wheels rest upon the ground. The result of this is to lower the piece and to increase the difficulties of handling the gun. By resting the wheels upon a hard and smooth sur- face this trouble is avoided. This surface is furnished, in field works, by wooden platforms, upon whicli the wheels of the gun-carriages rest. The platform is made rectangular or trapezoidal in plan, and large enough for the service of the gun. A field gun should have a platform at least ten feet wide in front, and fifteen feet long. A siege gun would require a platform fourteen feet wide and eighteen feet long. The dimensions, bill of materials, and method of constructing platforms, are laid down in the manuals of engineering and ordnance, and to these works the student is referred for the details of such construction. A temporary wooden phitform, which can be quickly constructed and can be used until a better one is pro- vided, may be made as follows : The earth upon which the platform is to rest should be thoroughly rammed. Trenches, as many as there are sleepers, are then dug parallel to the direc- trix of the embrasure, or perpendicular to the interior crest in case of a barbette. In these trenches sleepers are bedded, so as to have their rear ends raised about six inches above the front ends. Planks, at least three 134 FIELD FORTIFICATIONS. inches thick, are nailed or spiked transversely upop these sleepers. This practically completes the plat- form. A piece of timber, at least six inches in diameter, is spiked to the front part of the platform in such a position that, when the wheels of the carriage rest against it, they will not touch the interior slope of the parapet. This piece against which the wheels rest is called a hurter. It is frequently advisable to spike timbers along the sides, to act as guards and prevent the wheels of the carriage from running off the platform. These pieces, termed side-rails, are also useful to assist in holding the planks in place. The raising of the sleepers at the rear ends gives to the platform an inclination to the front, which assists in running the gun ''in battery," after it has been loaded, and assists in checking the recoil of the piece, when it is fired. The number of sleepers used for the platform de- pends upon the size of the platform and the abundance of timber in the vicinity of the work. A sleeper is laid directly under the middle line of the platform. The others are laid parallel to tliis one, on either side, and at equal distances apart. In emergencies, and where timber is scarce, only three sleepers may be used ; a middle one, and two outside ones, the latter being placed so as to be directly un- INTERIOR ARRANGEMENTS OF A FIELD WORK. V.\h der the wheels of the carriage, when the gun is '*in battery." If timber is plentiful, they are placed not more than two feet apart, from centre to centre, and sometimes even in juxtaposition. At the outer end of each sleeper a stout wooden picket is driven to keep the platform steady. In the Franco- Prussian war of 1870-1, the Prus- sians made use of a device which was sent into the field with each siege gun. This device consisted of two in- clined planes of stout plank, faced with sheet iron, with a rise of one on six. and from eight to nine feet long. An ordinary wooden platform, or merely a platform of planks, was laid, and on it and under the wheels were placed these inclined planes. II. Shelters for the troops, etc. 112. Shelters. — An efficient defence of a field work is greatly aided by shelters, arranged for the men and the stores, so that the men can rest in them, and the stores can be kept safe from the enemy's fire. The shelters generally used are known as bomb- proofs, and splinter-proofs, which differ from each other only in capacity and strength. Bomb-proofs must be strong enough to resist the effects both of .the impact and of the explosion of the projectiles which strike them. They should be roomy, and when used by the men, should be well ventilated. Splinter-proofs are so placed that they are not ex- 136 FIELD FORTIFICATIONS. posed to the impact of projectiles. They are liable to be struck by fragments of shells, or splinters knocked off by the impact of a projectile, and are therefore made only strong enough to resist the effects of the flying fragments and splinters produced by shells bursting, or by projectiles striking near them. 113. Construction of bomb-proofs. — Bomb- proofs may be built during the cotistruccion of the parapets, or after the parapets are finished. The latter is the more usual method. The position in a field work occupied by a bomb- proof depends upon the size of the work, the kind of trace, degree of exposure of the interior of the work, the convenience of the position, etc. Hence, bomb- proofs are sometimes placed under the parapet ; some- times in the gorge of a half-closed work; sometimes Fig. 43. in the middle of the parade, etc.; the position being determined by the circumstances of each case. Fig. 43 represents a cross section of a bomb-proof INTERIOR ARRANGEMENTS OF A FIEI4D WORK. 13' into which the men can retire and be safe from the effects of a direct plunging or curved fire. The construction of this bomb-proof was as fol- lows : A row of vertical posts, in contact or at short distances apart, were framed into ground sills, or set into the ground at the bottom of the trench, along a line far enough away from the side of the trench next to the enemy's fire to allow room for a man to recline at full length, or occupy a comfortable posi- tion. These posts were then capped with stout tim- bers. A second row of posts was placed, either vertical or inclined, against the side of the trench, and this row was capped in a similar manner. Cross timbers were laid in juxtaposition upon these capping timbers, and then covered with planks, fascines, etc., to form a tight roof. This roof, when thus formed, was covered with sufficient earth to make it proof against the effects produced by the impact of projectiles, or the effects resulting from their explosion. A thickness of five feet of earth, in a vertical direction, is usually con- sidered sufficient to make a roof proof against the effects produced by projectiles of field guns. Ingress and egress of tiie men using tlie bomb- proof may be facilitatod by cutting stoi)s into the side of the trench, as shown in the figure. The ])art of the bomb-proof resting against the side of the 138 FIELD FORTIFICATIONS, trench should be revetted by a covering of plank, fas- cines, or other suitable material, to keep the shelter dry, and to make it more comfortable. Guard beds should be constructed, when the bomb-proof is wide enough, so that the men can lie down at full length ; if not wide enough, benches can be made which will allow the men to assume easy positions. A construction like that shown in Fig. 43, can easily be placed under the banquette, when the com- mand is 9| feet, the banquette in this case serving as the top of the bomb-proof. 114. Blindages. — Any construction used in field works which has for its object the protection of the men and material against the effects of artillery fire from overhead, is termed a blindage. The pre- ceding construction, therefore, is a blindage. 115. Splinter-proofs. — Shelters which are not exposed to the impact of the projectiles of the enemy, need not be so strong as the bomb-proof. It will be sufficient if they are proof against the splinters and fragments of shells, produced by the enemy's fire. Shelters of this kind are usually constructed in inclined positions. (Fig. 44). They are made by plac- ing strong timbers, or bars of railroad iron, in an in- clined position against the surface to be protected, and in juxtaposition, and then covering them with earth sufficient to make the interior safe against the frag- ments which may strike the shelter. INTERIOR ARRANGEMENTS OF A FIELD WORK. 139 The inclination of the timbers will be equal to, or less, than the natural slope of the earth thrown against them. It is well to cover the pieces with raw-hides Fig. 44. or tarpaulins before the earth is thrown against them, to make the shelter water-tight. A thickness of two feet of earth is sufficient to resist the fragments of shells fired from field guns. In many cases the earth is placed upon the shelter by piling sand bags filled with earth against it. Entrance to the shelter is provided for by openings at the ends,, sometimes by openings left at intervals. Splinter-proofs, from their nature, are placed in those situations where they are not exposed to a direct fire. They are much used to protect doors, entrances, etc., which are exposed to the effects of bursting shells ; to protect vertical walls liable to injury from the same cause ; etc. 116. Powder magazines, etc. — Shelters in 140 FI^LD FORTIFICATIONS. whicli the ammunition and other stores can be placed and kept safe from the effects of the enemy's fire, are equally as important as the shelters for the men. The most important of these are the powder maga- zines, or those shelters intended for the storage of the ammunition. The rules given for the construction and location of bomb-proof shelters for men, apply equally to shel- ters of this class. The only difference in construction is in the size of the shelter, it being much smaller, as a rule, than that required for the use of troops. Large magazines are not constructed in ordinary field works. They take up too much room, and even the best of them are but poor places in which to Fig. 45. store ammunition for any length of time. The usual method adopted is to construct as many service magazines as may be necessary, near the guns to be served by them, making them large enough to INTERIOR ARRANGEMENTS OF A FIELD WORK. 141 contain the amount required for a definite service of the gun or guns to which they belong. 117. Service magazines. — Magazines of this kind are oftentimes built in the adjacent traverses (Fig. 45) if there be any ; generally under the parapet near the guns ; and sometimes under the barbettes. Fig. 46. The conditions to be observed in locating and con- Btructing a powder magazine are that it shall be con- veniently placed ; shall not be exposed to a direct lire 142 FIELD 'FORTIFICATIONS. of the enemy ; be made bomb-proof ; be well drained ; and if practicable, be well ventilated. A service magazine may be entirely above the sur- face of the ground (Fig. 45) or partly or wholly under the ground (Fig. 46). The construction of a service magazine, in which the magazine and passage-way are lined with wooden frames, is represented in Fig. 46. The frames are made of timbers or scantlings of the proper dimensions, each frame consisting of two uprights, called stanchions, a ground sill, and a cap. The interior dimensions of the frame are the same as that of the magazine, or six feet high and six feet wide, the least dimensions given, when practicable, to the width and height of the interior space. The frames are placed upright, about three feet apart, and in the position which they are to occupy. Their tops and sides are then planked over ; this plank- ing is called the sheeting. The bottom of the excavation is sloped from the sides to the middle, and from the rear to the front, to allow all water leaking through the magazine to col- lect in a shallow trench made along the middle line, and to run off into a drain prepared to receive it, or into a dry well dug near the entrance. The ground sills are then floored with boards. Great care should be taken to make the top water- tight, before the earth is placed upon it. This done, INTERIOR ARRANGEMENTS OF A FIELD WORK. 143 it is covered with several feet of earth depending upon the degree of exposure to which it is subjected. The plan, and horizontal section of tlie magazine and entrance, made by the plane A B ; '^n^ the eleva- tion, and section by the vertical plane, C D> i"*e shown in Fig. 46. The entrance to the magazine should be closed by a stout door, and the approach to it should be protected by a splinter-proof. If field artillery is employed to defend the work, the limber boxes are takea off and placed within the magazines. 118. Shelter for guns, etc. — Shelters are fre- quently provided for guns, implements, etc. The thing to be sheltered, its dimensions, and its uses, will regulate the details of construction of the shelter. The rules applicable for the shelter just de- scribed, apply equally to shelters of this class. 119. Materials used in the construction of shelters. — Timber has been considered to be the material used in the construction of the above shelters. This material is so abundant in the United States chat it can almost always be found in quantities near the work, and can be obtained quickly. It will therefore be the material chiefly used in temporary fortifications. No better material can be used for the transverse pieces of these shelters than railroad iron, if it can be obtained. The form of the rails allows the pieces to bo placed in juxtaposition without delay, and the strength 144 FIEIA) FO It TIFI CA TI XS. of the iron makes tlie roof better able to resist the shocks of the projectiles, and makes the structure more dur- able in its character. 120. Bomb-proof shelters used in the de- fences of Washington. — The field works employed to defend Washington, in the war of 1861-5, are fine examples of temporary fortifications. They were in the beginning, constructed according to the rules laid down for temporary works. But as time passed, and their great importance was recognized, the dimensions of the parts of tlie different works were increased, and the interior arrangements so improved that the works passed beyond the limits laid down for field works, and approximated in profile and in the interior arrange- ments to permanent fortifications. The section of a bomb-proof, shown in Fig. 47, gives a type of the shelters built for the men. Fig. 47. Its construction differs but slightly from those already described. It is larger, and is provided with a oanquette by means of which a musketry fire could be delivered over the top of the shelter, if required. INTERIOR ARRANGEMENTS OF A FIELD WORK, 143 The shelter represented by it was located near the middle of the parade. 121. Powder magazines used in the de- fences of Washington. — The powder magazines at drst ustrd were built like that described in art. 117. They were soon replaced by other constructions, of which the section shown in Fig. 48, represents the type. Fig. 48. The new magazines were made stronger and more durable, were better drained, and were well ventilated ; in all of which qualities the older ones were deficient. This particular example shown in Fig. 48, was built under a traverse. The essential difference between this particular magazine and those usually constructed, is the employ- ment of an air-chamber to prevent the accumulation of moisture which otherwise would take place upon the inner side of the magazine. This chamber was constructed by placing logs in an inclined position against the sides of the magazine, framing them into ii ground-sill, and bracing them at the middle. Small 140 FIELD FORTIFICATIONS, poles, from two to four inches in diameter, were laid against these inclined pieces, to hold up the earth thrown against them. Ventilators connected the in- terior of the magazine with the air chamber, and the air chamber with the outside. Particular attention was paid to making the roof water-tight in all the magazines. Whenever it was practicable, the drainage was car- ried to the main ditch of the work. On the sides of a magazine exposed to the enemy's fire, the thickness of earth was made such as to meas- ure at least ten feet from the woodwork of the maga- zine to the exterior, along a line making an angle of thirty degrees with the horizontal. Magazines of the dimensions of those built in the defences of Washington, are not usually required, nor are they' built in ordinary field works. But when a field work is to be occupied for some time, and a large quantity of ammunition is to be stored in temporary magazines, the principles governing the construction of the magazines should, as far as practicable, be in accordance with those illustrated by this example. 122. Position given to the shelters, etc. — No absolute rule, or set of rules, can be made which will apply to all cases in practice. Each particular work must be considered by itself and in connection with its surroundings. Interior arrangements, ex- tremely necessary in one particular work, might be INTERIOR ARRANGEMENTS OF A FIELD WORK. 141 useless in another ; the positions occupied by these arrangements in one work might be the worst places for them in a work of another kind, or in a work situated in a dijfferent locality. The plan shown in Fig. 49, representing the F»g. 49. northern half of a redoubt used at the seige of Pe- tersburg, in Virginia, in 1864, may be taken to illus- trate the way in which the interior arrangements are sometimes located. The trace of the redoubt was an irregular penta- gon, and the portion shown in the figure was that part toward the enemy. 148 FIELD FORTIFICATIONS. III. Communications, barriers, etc. 123. Communications. — The defenders of a closed work must have arrangements made by means of which they can enter or go out of the work when necessary. In case of continued lines, arrangements should be provided by means of which the defenders can make sorties. The method adopted is to leave openings in the parapet, through which passages are built leading to the outside of the work. These openings made in a parapet are necessarily weak points of a work. As a consequence, they should be placed where they will be the least exposed, and can be most easily guarded. No greater number should be constructed than the actual necessities of the work demand. In redoubts, the outlets are on the sides least ex- ))osed to attack ; in half-closed works, they are placed near the middle of the gorge ; in forts, they are usually placed near the re-entrants. A passage for the use of infantry only should not, as a geneml thing, be less than six feet wide; for artillery, not less than ten feet wide ; for sorties, the outlets in continued lines should be at least fifty yards wide. 124. Masks opposite the outlets. — The out- let should be masked in some way to prevent an enemy, on the same side of the work with the outlet, tSTERIOR ARtiANQEMENTS OF A FIELD WORK, 140 from seeing into the enclosed space. This is usually done by placing a traverse inside of the work, and oppo- site the outlet (Fig. 50). Fig. 50. The traverse has, ordinarily, the same height as the parapet behind which it is placed, and a similar profile. Room sufficient for a road-way, is left between the foot of the exterior slope of the traverse and the foot of the banquette slope of the parapet. The trav- erse must be long enough to intercept all projectiles which an enemy on the outside can fire through the opening in the parapet. Its length will therefore de- pend upon the width of the outlet and the thickness of the parapet. The limiting plane of fire, above which a direct fire may be neglected, is taken to be five feet above 150 PlELD PORTlPtCATtom, the ground un which the man stands. The intersec- tion of this plane of fire with the sides of the outlet should be determined, and the extreme lines of fire drawn. Let the dotted lines, through c and g, be the extreme lines of fire. If the exterior crest of the traverse rests on these two lines, the traverse will in- tercept all fire from the outside coming through the opening in the parapet. When the height of the parapet is eight feet, it will be sufficiently accurate for all practical purposes to take the horizontal plane passing through the exter- ior crest as the limiting plane of fire, and to use the horizontal lines passing through c and g, shown dotted in the figure, as the extreme lines of fire. The more accurate method would be to determine these extreme lines, by passing a plane five feet above the ground, and finding its intersection with the sides of the outlet, and with the exterior and interior slopes of the parapet. Join, by a straight line, the point of intersection of the line cut from the exterior slope and either of the lines cut from the sides of the outlet, with the point of intersection of the line cut from the opposite side of the outlet and the line cut from the interior slope. This line, thus drawn, will be one of the limiting lines. In a simi- lar way, the other line could be obtained. 125. The length of the traverse may be- shortened Dy turning back the interior crest at right angles to its tyTEinon ARRAxaJSM^NTs of a FiELiy wohk. 151 general direction, and extending it as far as the crest of the banquette (B B, Fig. 50). Instead of liaving a road along its entire front, the traverse is sometimes joined to the parapet on one side of the opening, as shown by the dotted lines h d and e /, in Fig. 50. Sometimes, it is necessary to dispense with a tra- verse in the interior. Especially is this the case where the outlet is a very wide one, and the interior space quite limited. The method adopted to mask the interior of the work in this latter case, is to place the traverse op- posite the outlet on the outside, and beyond the ditch. (Fig. 51). Fig. 51, Interior Crest L.- .J». j The traverse in this case is usually broken, gener- ally a redan in trace, with the profile of a parapet, but commanded by the parapet in rear. 126. Barriers. — The outlets are usually ar- ranged so that they can be quickly closed, to guard 152 FIELD FOR TIFICA TIONS. agtiinst surprise. The means used is a gate, techni- cally termed a barrier. The gate is made with two leaves, hanging on posts by liinges, and made to open inward. The frame of each leaf is composed of two uprights, called stiles j two cross pieces, one at the top and the other at the bottom, called rails • and a diagonal brace, called a swinging bar. The leaf of the barrier may be made open, by spik- ing stout upright pieces, with intervals between them, to the pieces of the frame ; or it may be made solid, forming what is known as a bullet-proof gate. (Fig. 52). Fig. 52. Since the gate must be strong, the leaves of it are necessarily very heavy. The leaves must be hung upon stout posts, firmly braced into the ground, to sus- tain the great weight of the gate. The top rails of all barriers should not be less than six feet above the ground. INTERIOR ARRANGEMENTS OF A FIELD WORK. 153 III the barriers with open leaves, the vertical pieces are usually extended from eighteen inches to two feet above the top rails, and their upper ends sharpened. In those which are solid, it is usual to arrange some obstruction upon the top rail, such as sharp pointed spikes, broken glass, etc., to interfere with persons climbing over the top. It is usual to provide aper- tures in the leaves, through which the men can fire upon the ground on the outside. 127. Bridges. — When the ditch has been com- pleted along that part of the work in front of the out- let, it is usual to carry the roadway across the ditch by means of a bridge. The ditches of field works are, as a rule, quite nar- row, and the bridges used to span them are very sim- ple constructions. A common method of building the bridge is to lay three or more sleepers across the ditch, and cover them with planks laid transversely. If the span is sufficient to require intermediate supports, these are obtained by using trestles placed in the ditch. A bridge built in this way can be quickly re- moved and speedily re-built, if there be any necessity for it. Hand-books on military engineering describe a number of bridge constructions for use in fortifica- cations. These bridges are all arranged so as to ad- mit of communication across the ditch being inter- io4 FIELD FORTIFICATIONS. rupted at pleasure, aud are either draw or rolling bridges. The draw-bridges are usually made with a leaf to revolve around a horizontal axis, the leaf usually be- ing raised to a vertical position when the communi- cation is interrupted. The rolling bridges are ar- ranged to be pushed out from the work, and drawn back into it. Bridges of this class, known as mova- ble bridges, are useful to guard against surprise, to prevent stragglers from entering, and to keep the gar- rison in the work. As a defence against an assault of a field work, they are of but little value. The best method, is to have no ditch in front of an outlet, but let the roadway be on the natural surface of the ground. 128. Ramps. — The short roads used in fortifica- tions to ascend from one level to another, are termed ramps. The width of a ramp depends upon its use, fol- lowing the rule laid down for the width of passages. A width of six feet for infantry, and of ten feet for artillery, are the widths generally used. The inclination of the ramp may be as great as one on six, and as little as one on fifteen, depending upon the difference of level between the top and bot- tom. The side slopes are of earth with its natural slope. The ramps in a work should be placed in posi- INTERTOn AtiRANGBMBNTS OF A FIELD WORK. 155 tions where they will not be in the way, nor occupy room which may be required for other purposes. Steps or stairways are sometimes used instead of ramps. The rule for them is that the breadth of each step, called the tread, shall be at least twelve inches, and the heigiit of the step, known as the rise, shall be about eight inches. They are substituted for ramps in those places where there is not sufficient room for the ramp. IV. Arrangements Intended for the Comfort and Health of a Garrison. 129. Nature of the arrangements. — A garri- son compelled to live within an enclosed space like a field work, should be provided with all the arrange- ments which are necessary for the health and for the comfort of the men, consistent with surrounding circumstances. The arrangements essential to the health and comfort of the men include those intended to pro- tect them from the weather, to provide for their support, and to supply their necessities. The principal arrangements are the tents, huts, or shelters in which the men are sheltered ; the guard-houses, and rooms for those on duty ; the kitchens and bake-ovens in which the food is pre- pared ; the sinks or privies, and the places provided for the men for washing themselves and their cloth- 156 nULD FORTIFICATIONS. ing ; the hospitals for the sick ; etc. Wells, or means of providing the garrison with a supply of good drinking water, form no unimportant part of the arrangements necessary for the comfort as well as the health of a garrison. The limits of this book will not admit of a dis- cussion, nor even a reference to the various divisions, of this important section of the interior arrangements of a field work. These arrangements are second only to those re- quired for actual defence, and in many cases they are equal, as the defence of the work in a great measure depends upon them. The only rule that can be laid down is to make all these arrangements of a temporary character, and to place them so that they can be removed, at a moment's notice, out of the way of any interference with an active defence of the fortification. V. Other kinds of Interior Arrangements. 130. Secondary interior arrangements. — Besides the interior arrangements which have been described, or mentioned, there are others which are secondary in their nature. These are the arrange- ments which are to be used under certain contingen- cies, or in cases of emergency. An example, would be a defence placed within a field work, which defence INTERIOR ARRAXUEMEXTS OF A FIELD WORK. 151 can be used only when the main work is no longer defensible, etc. 131. Block-houses. — It is frequently the case that a separate fortification is constructed, lying en- tirely within a work exterior to it, into which a gar- rison can retire and protract their resistance, even after the outer fortification has been taken, or has been made unfit for further defence. If this interior work is a line of earthern para- pet, it is called a retrenchment • if it is a defen- sible building, it is termed a keep. The term, keep, is also applied to a work which is entirely separate and distinct from the work exterior to it, whatever may be the material used in its con- struction. In a field work, the keep is built of timber, and is called a block-house. The conditions wliich should be fulfilled by a block-house intended for a keep, are that it shall have a good command over all of the interior space enclosed by the outer work, and shall occupy a po- sition such that all parts of the exterior work can be seen from it. The plan of a block-house is selected by the same general rules wliich are used for selecting the trace of a field work. It may be square, rectangular, oc- tagonal, and even cruciform, in plan, according to tlie locality in whicli it is placed and the fire which it has to deliver. 158 FIELD FORTIFICATIONS, The dimensions of a block-house should be suffic- ient to allow sleeping accommodations for the men who are to occupy it; and in some cases allowance should be made for other accommodations. Its interior dimen- sions should give at least a height of six feet in the clear for the rooms ; a height of eight or nine feet gives better accommodations and better ventilation. The width of the interior should not be less than nine feet in the clear, as this is the least distance which can be used and give room for a passageway and a row of bunks. The length of a block-house will depend upon the number of men it has to accommodate, after the width has been assumed. Block-houses must be made strong enough to re- sist the projectiles which may strike them and should be proof against fire and splinters. They should be free from dampness, and should be well ventilated. The conditions given for a bomb-proof are ap- plicable to the block-house, with the additional one of arranging its walls for defence. This is fulfilled by perforating the walls with loop-lioles for musketry and embrasures for cannon, when the latter are used. The example given in Figs. 53, 54, and 55. shows the details of construction of a wooden block-house, used in the late war in the United States. INTERIOR ARRANGEMENTS OF A FIELD WORK. 150 The Willis were made of two thicknesses of logs (Fig. 53) the inner row being vertical, the outer row being horizontal. Fig. 53 Fig. 54. wmfffmmmm}^ The inner row was composed of logs in contact, framed into a ground-sill, and capped by a heavy piece of timber. The logs in the outer row were laid one upon the other, having a surface of contact of at least eight inches. A roof of logs was laid upon these rows forming the walls, and then boarded over to make it water- tight, 160 FIELD FORTIFICATIONS. This roof was covered with eartli from three to four feet deep, to make it fire and splinter proof. The sides exposed to the enemy were banked with earth (Fig. 53) to increase their resistance to the enemy's projectiles. Loop-holes were constructed through which the defence could fire upon the ground exterior to the block-house. 132. Isolated block-houses. — Timber block- houses were used frequently in the war of 1861-5 in isolated spots, as independent works. In these places, they were, as a rule, exposed to attack only from infantry or cavalry, or a few pieces of field artillery. The construction shown in Figures 53, 54, and 55 is a type of these isolated block-houses. It was found from experience that it required a thickness of forty inches of solid timber to resist the projectiles of field-guns. These isolated block-houses were frequently built two stories high. The upper story was usually placed so as to have its sides make an angle with the sides of the lower story. By this arrangement, the corners of the upper story projected over the sides of the lower. This arrangement of the upper story removed the dead space near the sides of the lower story, and the sector without fire in front of the angles. Block-houses ex- nosed to artillery fire should not have a second story. INTER l< Hi Ai:RAy(iEMK.\TS OF A FIELD WORK. IGl 133. stockades. — A line of stout posts or trunks of trees firmly set in the ground, in contact wicli each other, and arranged for defence, is caller"' a stockade. A stockade is used principally when there iv plenty of timber and little or no danger of expos- ure to artillery fire. It is frequently used to clos^ the gorge of a field work, and to guard against the work being carried by a surprise, by bodies of in- fantry attacking the work in rear. The timbers of a stockade may be either round or square. If round, they are hewed to a flat surface on two of the sides so that the posts, when placed in position, shall have a close contact of at least four inches. The top of a stockade should be at least eight feet Fig. 56. Fig. 67. above the ground on whioli it is placed, and it should have the upper ends of the timbers sharpened, or ar- ranged with spikes, or fixed in some way to offer an 162 FIELD FORTIFICATIONS. obstruction to climbing over the top. A stockade is arranged for defence by cutting loopholes (Figures 5G, and 57,) which can be used by a soldier when firing. The height of the loop-hole may be just four feet and six inches above the ground, or higher. In the former case, the soldier stands upon the natural surface of the ground (Fig. 56) in the act of firing ; in the latter case, he stands upon a banquette of earth (Fig. 57) or some temporary arrangement which raises him to the proper height. The exterior opening of the loop-hole should be not less than six feet above the ground on which the enemy may stand when he is close to the stockade, so that he can not make use of the loop-hole. Some obstruction must therefore be placed, in front of the Fig. 58. loop-hole, keeping the enemy away from it ; or the ground immediately in its front should be deepened by digging a trench (Fig. 56). If the loop-hole can be placed six feet above the INTERIOR ARRAyUEMENTS OF A FIELD WORK. 103 ground, it will be practicable to arrange the loop- holes so as to furnish a double tier of fire (Fig. 58). The loop-holes of the lower row should be arranged so that they can not be used by the enemy. They should not be higher than eighteen inches above the ground on the outside ; on the inside, the ground must be cut away (Fig. 58) or a trench dug in rear of the stockade, so as to allow the use of the loop-holes by the defence. The construction of a loop-hole in a stockade can be understood by examining Fig. 59. The exterior opening of a loop-hole is represented by a; the interior opening h\ h. A side view is rep- resented by the longitudinal section c. The same rules govern the construction of loop- holes when there is time to execute them, as are used in constructing embrasures in a parapet. The exterior opening is made as small as possible and serve its purpose. An opening of two inches wide and five inches long is considered sufficiently large for the rifled musket of the present day (Art. 108). 164 FIELD FORTIFICATIONS. The size of the interior opening will depend upon the field of fire. A width of six inches and a height of twelve inches are the dimensions usually given to the interior opening, when the timbers of the stock- ade are twelve inches thick. When there is a great hurry, a notch may be cut by an axe or a saw in each of the logs, which will do for a loop-hole. The front of this notch is shown by the figure d, and the side view by the figure /. A perpendicular cut of four and a half inches is made in each log at the point which is to be the bot- tom of the loop-hole. An oblique cut is then made about nine inches long. The two logs placed in con- tact form a loop-hole as seen in d (Fig. 59). The exterior opening may be reduced afterwards by spiking a piece of iron upon the logs. Kailroad Fig. 60. i ii 11 ii n 1 chairs, fish-plates, etc., frequently found in the vi- cinity, can be used for this purpose. Sometimes the stockade is strengthened by a second row of timbers. The method is shown in Fig 60. INTERIOR ARRANGEMENTS OF A FIELD WORK. ICo This figure shows also the method of making loop- holes, by cutting away a strip from two adjacent posts, leaving an interval through which the men can fire. Loop-holes are usually made aboat two feet and six inches apart CHAPTER XII. ARRANGEMENTS EXTERIOR TO THE PARAPET. 134. Kinds. — There are two kinds of arrange- ments employed on the exterior of a parapet to add to the strength of a work, viz : the arrangements made to defend the ditch, and those made to obstruct an enemy's approach. The term, ditch defence, is used to designate the arrangement made exterior to the parapet, by which a fire is made to sweep the ditch. The term, obstacle, is applied to any construc- tion or arrangement, whatever may be its nature which, by its passive resistance, obstructs the ap- proach of an enemy advancing to assault the work. Hence, anything is an obstacle which diverts the at- tention of the enemy from the assault to the imme- diate surroundings of himself. I. Ditch defences. 135. Classes of ditch defences. — The ditch is best defended, as a general rule, by the work itself. But as a flanking disposition is not always attainable, and as it is not usually practicable in field works, some arrangement must be provided by means of which the ARRANGEMENTS EXTERIOR TO THE PARAPET, 1G7 ditches can be swept, in those works where it is neces- sary that the dead spaces should be removed. The constructions used for this purpose are either caponnidres or galleries. 136. Oaponni^res.— A caponni^re is a bomb- proof construction built in the ditch (Fig 61). Fig. 61. ^ Superior Slope It may be arranged for a fire in two directions, or only in one. The amount of fire fixes its length. When arranged for a fire in two directions, it should be wide enough to accommodate two ranks, facing in opposite wjiys. This width should not be less than eight feet. A width of four feet and six inches is suflScient for one rank. The interior height should not be less than six 168 FIELD FORTIFICATIONS. feet in the clear (Fig. 62). The bottom of the capon- ni^re may be on the same level, as the ditch, or be- low it. Fig. 62. 'mm^^mmvvy/^ilVfW The loop-holes are arranged as described in art. 133. The top should not rise above the crest of the counterscarp and should be made bomb-proof. Entrance is obtained by a covered passage, P, leading under the parapet. It is constructed by using frames like those described in Art. 117. The construction of the roof and the position of the loop-holes are shown in Fig. 62, which represents a cross section made by the plane A B. in Fig 61. 137. Galleries. — Galleries used to sweep the ditch with their fires are constructions similar to ca- ponni^res. They receive the name of scarp or counterscarp galleries, according to the position which they occupy. The usual method of building a gallery is to ex- cavate the earth behind the scarp or counterscarp, ARRANGEMENTS EXTERIOR TO THE PARAPET, 1G9 as the case may be, until there is room to place the frame, if there is one to be used, or until there is room to accommodate the men who are to occupy the gallery. This excavation is then closed in front by a stockade built along the line of the scarp or counterscarp ; it is closed overhead by a bomb-proof roof, the top of which is the parapet, in the case of a scarp gallery, and the glacis or natural surface of the ground, if a counterscarp gallery. The arrangements of loop-holes, dimensions, etc., are all similar in kind and character to those men- tioned for caponni^res. Entrance to a scarp gallery is by a covered pas- sage ; the same might be used for a counterscarp gallery ; generally, the entrance to the latter is by openings into the ditch, at the ends of the gallery, (vhich openings can be closed by bullet-proof doors. Counterscarp galleries are used at the salients of a work ; scarp galleries at the re-entrants. Scarp and counterscarp galleries, as well as capon- ni^res, are, in their details of construction, nothing but Btockades. It follows then that all remarks relating to stockades apply equally to these ditch defences. They should not be placed in positions where they would be exposed to artillery fire. If there was danger of exposure to this fire, and it was ne- cessary to use one of these defences, it is plain that the counterscarp gallery is to be preferred. 170 FIELD FORTIFICATIONS. It may be remarked, however, that ditch defences of this kind rarely repay, by their advantages, the amount of time and labor expended in their con- sti'uction; and then, only in the case of a large en- closed work with deep ditches. 11. Obstacles. 138. Kinds. — Obstacles are of two general kinds, viz : natural, and artificial. Marshes, water courses, precipices, etc., are exam- ples of obstacles of the first kind. A very little labor, well applied, will often convert these natural features of the ground into serious obstacles to an enemy's advance. Ditches, abatis, palisades, slashings, etc., are ex- amples of artificial obstacles. They can be made important adjuncts of a defensive work, and can be made to perform effective service in the defence of any position. 139. Ditches. — The ditches, from which the earth is obtained to make the parapet, can be made into obstacles to an enemy's assault. To offer an obstruction difficult to surmount, a ditch should be not less than six feet deep and twelve feet wide. (Art. 28). The difficulties of surmounting an obstruc- tion of this kind can be greatly increased by making the scarp steep, and by a Judicious arrangement of ARRANGEMENTS EXTERIOR TO THE PARAPET. 171 some of the other obstacles, hereinafter named, in and along the ditch. When the scarp is made steep, it is necessary to revet its surface, so as to protect it from the weather and to hold the earth in place. In the late war in the United States, the revetments used for the scarps of works when strength was required, were made of timber or plank. The timber scarp revetment (Fig. 63) was com- Flg.64. Fig. 63. posed of logs placed in contact with each other, with a slight slope, about ^, and capped by a log notched on the uprights ; this capping log being tied back to an anchor log, bedded into the natural soil md held in place by pickets. The plank scarp revetment (Fig. 64) differed but 172 FIELD FORTIFICATIONS. slightly from the timber revetment. The posts, in- stead of being in contact with each other, were placed from six to eight feet apart, and the earth was lield up by planks placed on edge behind the posts. These revetments are practically the same as the revetments described in articles 81 and 82. The timber revetment is the stronger of the two, but requires more time, labor, and timber to construct it than the plank revetment. 140. Abatis. — A row of large limbs and branches of trees, with the ends of the branches sharpened, ])laced with the points towards the enemy, forms the obstacle known as an abatis. An abatis may occupy an upright position in a ditch (Fig. 65) or it may lie in a horizontal position Fig. 65, on the ground in front of a work (Fig. (jQ), The latter is the usual position given to an abatis. An abatis, to be an efficient obstacle, should con- sist of stout limbs, twelve or 15 feet long, laid as ARRAXGEMENTS EXTERIOR TO TBE PARAPET. 113 close togetlier as possible, with the butts secured by stout stakes, and should not be exposed to the enemy's artillery fire. When laid in front of a work, it ia usual to protect it by a slight glacis. (Fig. 66.) Fig. 66. 141. Entanglement. — An entanglement made by driving stout stakes into the ground from six to eight feet apart and connecting them by stout wire twisted around the stakes, forms an excellent obstacle. (Fig. 67.) It is quickly made when the materials are close at hand. Fig. 67. 142. Chevaux-de-frise, — A cheval-de-frise (Fig. C8) is a square timber, perforated with holes in which sticks shod with iron are inserted. The holes are placed about six inches apart, and are large enough to admit a stick of at least two inches in diameter. in PIELD FOR TIFICA TtONS. These sticks, called lances, are ten feet long, and are made to project equally on the two sides of the body, as the timber is called. The length of each cheval- Flg.68. de-frise is from six to ten feet. They are fastened together at the ends by stout wire, or by chains. In the British service, portable chevaux-de-frise made of iron, and in lengths of six feet, are used. 143. Palisades. — A palisading is simply a fence, made of strong and stout poles or pickets firmly set into the ground (Fig. 69). The poles are placed in a vertical or slightly in- ARRANOmrEKTS t:XfEtiton TO TttE P AH A PET. Ito clined position, with intervals between them of about three inches. The poles are obtained by sawing trunks of trees into lengths of from ten to twelve feet, and splitting them into rails of not less than four inches thick. The palisade is formed by digging a trench three feet deep, planting the rails in it, and ramming the earth around them. The rails are fastened together by being spiked or nailed to a stout riband piece placed about a foot below the surface of the ground. A second riband piece is used about a foot below the top of the rails. It is usual to sharpen the rails to a point to add to the difficulties of climbing over the palisade. Fig. 70. 144. Fraises. — A palisading occupying a hori- fontul position (Fig. 70) is called a fraise. 145. Small pickets. — Small straight branches of hard and tough wood are frequently cut into short HO PtBLD FORTinCATIOKS. lengths, driven firmly into the ground, leaving from one to two feet projecting, and pointed at the upper end. Obstructions of this kind are called small pickets (Fig. 70). 146. Crow's feet, harrows, etc. — Obstacles ol other kinds having the same object in view as small pickets are sometimes used. The most noted are the obstacles known as crow's feet ; ordinary harrows turned upside down with the teeth upvvards and the frames buried ; planks with spikes driven into them, placed so as to have the points upwards ; etc. A crow's foot is made of four stout iron spikes, welded together at their heads, and arranged so that in whatever position the obstacle is thrown upon the ground, 'one of its points shall stand upwards. 147. Military pits. — Excavations made in the ground, conical or pyramidal in form, with a small picket driven into the bottom, are called military pits. (French, trous-de-loup.) They are of two kinds, viz : deep and shallow. Deep military pits (Fig. 71) should not be less than six feet in depth, so that if they fall into the possession of the enemy, they can not be used against the defence. They are usually made about six feet in diameter at top, and about one foot at the bottom, and are placed so that the centres shall be about ten feet apart. They should be placed in rows, at least three in number, the ARRANGEMENTS EXTERIOR TO THE PARAPET, 177 pits being in quincunx order. The earth obtained by the excavation, should be heaped up on the ground be- tween the pits. Fig. 71. Shallow pits should not be deeper than about two feet, so that the enemy could not obtain shelter by getting into them. They should cover the ground in a zig-zag ar- rangement, the upper bases being made square or rectangular in form, and in contact with each other. The side of the upper base should be made about equal to the depth of the pit. The earth obtained from the holes is thrown in front of the arrangement, makinGf a glacis. 148. Slashing. — In compliance with the princi- ple that all houses, trees, brushwood, etc., within range of the work, which could be used as a shelter and a place of concealment by the enemy's sharpshooters, 178 FIELD FORTIFICATIONS, should be removed, it is essential that the trees within six hundred yards of the work should be cut down. As it is not practicable to remove immediately the trees from the spot, it is the custom to cut them down so that they shall form, while lying on the ground, an obstacle which may be used in the de- fence of the work. Trees cut down so as to fall in all directions, 'orm what is known as a slashing. It is better, where the trees are intended to be used as an obsta- cle, that they be cut so as to fall towards the enemy ; and, in the case of the smaller trees, which might be moved by a few men, the trunks should not be cut entirely through, but only enough to allow the trees to fall, (Fig. 72.) Fig. 72 ^^^j^ar- A thick and well arranged slashing forms an ex- cellent obstruction to an enemy's free movements. It has the serious defect of being easily burned when dry. 149. Torpedoes. — Loaded shells buried in the earth just deep enough to be concealed, and ar- ARRANGEMENTS EXTERIOR TO TBE PARAPET, 179 rimged so that they can be exploded automatically, or at the will of the defence, have been used as obstacles. Arrangements of this kind are known as torpedoes. The case enclosing the charge may be either of wood or iron. Condemned shells are especially suita- ble for the purpose. The explosive compound used to charge them may be powder, gun-cotton, nitro-glycerine, or any material which, upon being fired, will burst the case containing the charge and scatter the fragments in every direction. The automatic — sometimes known as the *' sensi- tive" torpedo — is fired by contact. It has the advan- tage of being exploded at the right time, but has the disadvantages of making the ground, in which it is buried, dangerous to the defence, and of subjecting the men when handling it to the danger of acciden- tal explosions. The torpedo which is fired " at will " has the dis- advantage of being fired oftentimes prematurely, or when it is too late. Circumstances can only decide as to which of the two is to be preferred as an obstacle. 150. Stone-fougasses. — Military mining is rarely resorted to either in the attack or the defence of a field work. When used, it forms part of the operations of a siege, which may sometimes be under taken to get possession of a field work. 180 FIELD FOUTIFICATIONS. A mine placed at the bottom of a small shaft anil fired by a fuze, is termed a fougasse. A particular arrangement known as the stone fougasse is always described by military writers on the subject of field works as a useful device in the defence of a work. It is usually constructed by excavating a funnel shaped hole in the ground to a depth of five or six feet. The axis of the excavation has an inclination of about 40° with the horizon ; the lower line has an inclination of about 30°. A box, containing the powder, is placed in the bot- tom of the hole, and covered by a wooden shield which exactly fits the excavation. The earth of the excava- tion is well rammed around the shield on top and behind, so as to make the line of least resistance offered to the explosion coincide as nearly as possible with the axis of the excavation. On this shield and in this hole are poured from three to five cubic yards of stones, the smallest weighing one pound. Some- times loaded shells are mingled with the stones. The fougasse is fired by means of a fuze, or by electricity, in a similar manner to that used for firing mines. The result of the explosion is to scatter the stones in a shower over a considerable extent of ground in front of the fougasse. The whole construction is simply an extemporized ARRANGEMENTS EXTERIOR TO THE PARAPET, 181 mortar for throwing stones, capable of a single dis- charge. Its position must be concealed from the enemy's view. 151. Shell-fougasses. — A shell fougasse is a box containing loaded shells, concealed in the earth, and so arranged as to be exploded when the enemy is over the spot. The box is divided by a partition into two parts, an upper and a lower. The loaded shells are placed in the upper part, with the fuzes downwards and con- necting with the lower part by holes bored in the partition. A charge of powder is placed in the lower division of the box of sufficient quantity, when fired, to throw the shells to the surface. This charge is fired by means of a fuze, or by electricity, like other fougasses. 152. Inundations. — If the position occupied by a field work is near a stream, it may be possible, in some cases, to increase the difficulties of the enemy's approach by inundating the ground over which he has to pass. The inundation may be produced by building dams and causiug tlie waters of the stream to overflow its banks. If the depth of the water over the approaches is greater than five feet, the obstacle may be considered as practically insurmountable. If the depth is less, the obstacle is still a serious 182 FIELD FORTIFICATIONS. one, and ciin be made greater by digging pits and ditches at random and having them covered with a sheet of water. The ditches of the work, already im- portant obstacles in themselves, can, by flooding them, be made almost impassable. 153. Uses of obstacles. — No obstacle is in- surmountable. Obstacles may hinder, and even stop for a while, an enemy^s approach, but they can be over- come. Their passive resistance must be aided by the active resistance of the defence. These obstacles, in order that they should be ac- cessory means of defence, should detain the enemy in a position where he will be under the fire of the defenders at close range. Hence, the following con- ditions should be observed in arranging the obstacles in front of a field work. 1. The obstacles should be placed within close musketry range of the defence. 2. They should be arranged so as not to afford shelter to the enemy. 3. They should, as a rule, be protected from the fire of the enemy's artillery. 4. They should be arranged so as not to interfere with an active defence of the work. Their uses as obstacles will depend upon the de- gree of resistance which they offer in harmony with the foregoing conditions. Abatis placed in the ditch will, in one case, be in ARRANGEMENTS EXTERIOR TO THE PARAPET. 180 the best position ; in another, it should be placed some distance in front of the work. A fraise placed in the scarp, when the ditch is swept by a fire from the work or from ditch de- fences, will be better than if along the counterscarp. Torpedoes, military pits, entanglements, etc., may all be combined. In some cases, the ground in front of the work will be the better position ; in others, the crest of the counterscarp and the ditches offer the best conditions for their use. As a general rule, it is advisable to place the ob- stacles not nearer than fifty yards to the interior crest, if the profile is a weak one. When the profile is strong, it is not a matter of so much importance, so long as the assaulting columns are exposed to the fire of the defence. It is well to remark with respect to inundations, that they should not be used until the last moment. The unhealthiness due to the presence of stagnant water is apt to produce more casualties than are to be feared from the enemy's attacks. If the dams can not be protected or hidden from the enemy's artillery fire, they should be built, as far as possible, so that the enemy can bring his fire to bear only upon the upper side. The amount of the dam exposed to his fire will then only be the portion between the top and the surface of the water. CHAPTER XIII. FIELD FOETIFICATIONS UPOJS" IRREGULAR SITE8. 154. General considerations. — In the preced- ing chapters, the site of the field work and the ground in its immediate vicinity were considered as practically level. The case now to be considered is the one in which the site and the ground in the immediate vicinity of, the work are irregular in formation. It has been seen how the trace and the profile of a work are modified by the presence of neighboring heights, which command the work. (Chap. X.) It is now to be shown how the irregularities of the site modify the profile and trace of a work. A compliance with the first principle given for fortifications (Art. 7) would require that the work be placed upon the high ground, rather than upon the low ; a compliance with the second principle would require that the parapets be placed near the brow of a height and not away from it. Hence, it is seen that the trace of a work generally follows the brow of the high ground forming its site, and is placed so that the work can sweep with its fire? the slopes in front of the parapet. FIELD FORTlFICATIOyS UPON IRREGULAR SITES. 185 165. Three cases will arise. The case in which the slopes of the site are gentle, easily ascended, and can be swept by front and direct fires of artil- lery. The case in which the slopes can be ascended by infantry without difficulty, but are too steep to be swept by a direct artillery fire. And the case where the slopes are so steep as to be ascended only by climbing. The first case, or when the slopes are gentle, is the one most frequently under consideration. 156. Modification of profile. — Suppose a field work to occupy a surface which is level or slightly undulating considerably above the surrounding coun- try, but joined to it by gentle slopes. It will be easily seen that a change of position in the lines of a work upon this plateau can only be made by modifying the profile. Take a site like that shown in Fig. 73, and sup- pose the general directions in which a strong direct fire must be had (second principle of trace, Art. 30), to be indicated by the lines X Y, Xi Yi? '^^^^ Xs Y^-'. (Pig. 73). The lines A D, A B. nnd B C, drawn per- pendicular to the lines just named, would indicate the directions that the interior crests should have to get these direct fires. The next ]>o;nt is the placing of tliose lines so that the fire from them shall sweep the slopes ir their front. 186 FIELD FORTIFICATIONS. Suppose a profile, X' Y' (Fig. 74) to be made representing the section cat out by the plane X ¥• (Fig. 73.) SoaletltoflO This profile gives the inclination of the slope coc- necting the lower level with the high ground. An examination of the profile makes it plain that a work placed on the plateau, B H; should not be too near FIELD FOUTIFICATIONa UPON IRREGULAR SITES. 18^ B, nor too far away from it. If too near, the guru can not be depressed enough to sweep the slope, B F* If too far, the brow, B> of the high ground will in- tercept the shots and make a dead space of the slope. If a straight line, p B> be drawn tangent to the slope, or so that no point of it should be more than eighteen inches from the ground, and the interior crest of the work be placed upon it, it is plain that the whole slope can be swept by a direct fire of the work, so long as the angle made by this line witli the horizontal is not greater than that made by the superior slope of the parapet. If the face, A B (Fig. 73) occupies such a posi- tion that the interior crest, eight feet above the ground on which it is placed, is in this line, p P (Fig. 74), then it is evident that the slope can be thor- oughly swept by" its fire if the incHnation of the slope is not greater than that of the superior slope. It will also be seen, if the vertical at P (Fig. 74) represents this position of the interior crest, that moving P nearer to B can only be effected by lowering the parapet ; or, moving it away towards H> by rais- ing the parapet ; otherwise the slope would not be swept by a direct fire from the face A B. The foregoing sufficiently explains how the profile may be modified, with respect to the height of the parapet and inclination of the superior slope, for the purpose of obtaining a direct fire. 188 FIELD FORTIFICATIONS. 157. Modification of trace. — It is not always practicable, by moving the parapet, to arrange the trace of a work so that a direct fire can be obtained upon the slopes in its front. This is particularly the case in steep slopes, and upon ground in which the contours are very irregular in form. Slopes un- seen by direct fire may be swept sometimes by a flanking fire from other parts of the work, or other works built for the purpose. Circumstances will decide as to the method which will best effect the object. As an example, take the case of a piece of ground with an irregular outline, like that shown in Fig. 75. .The plan of laying out the trace parallel to the brow would not be a good one, because of the great FIELD FORTIFlCATlOys UPON IRREGULAR SITES. 189 development of interior crest which would result, and even with that, a failure to obtain all the direct fire needed. A better plan would be to modify the trace, and to make use of a line Avith intervals. A line of works like Bj A« etc., might be placed upon the salient spurs and in defensive relations with each other. A second line, as R, etc., might be used to defend the ground between the works in the first line, and to reach points not seen by the fire from these works. Or, if it is desirable to use a " continued line," and it is not expedient nor judicious to follow the brow of the heights, a trace like that shown m Fig. 76 may be used. The general direction in which the fire of the enemy is to be expected is shown by the arrow. 190 FIELD FORTIFICATIONS. The trace, instead of following the brow, runs from A to B along ground considerably lower than these points. A strong work should be built upon each of the points A ^^^ B^ and an indented line receding, as it descends the slope, should join these works. The long branches should be directed so as to be safe from enfi- lading fire, and the short branches arranged to bring flanking fires upon the slopes in front of the faces. The advantages of such a line when compared with a straight line joining the two points, A and B> are manifest. 158. Defence of steep slopes. — The brow of the height where the slopes are too steep for direct artillery fire may be broken, forming spurs ; or the brow may be regular in form. In the former case, works are placed upon the spurs, and the slopes in front of the works are swept by fires from the adjacent works, or by auxiliary works constructed for the purpose. In the latter case, the works should be arranged to bring as strong an artillery fire as practicable upon the approaches to the foot of the hill, and supplemental flanking arrange- ments should be used to sweep the slopes with mus- ketry fire. Under circumstances like these, the works are not usually exposed to a close artillery fire, and they are not therefore required to be so strong. The profiles may be modified accordingly. FIELD FORTIFICATlOys Ul'OX IRREG ULAR SITES. 101 159. Defence of precipitous slopes. — When the slopes are so steep us to be ascended only by climbing by using the hands in the ascent, their defence is an easy matter. The works to be constructed, unless exposed to cannon fire, may have a very slight profile. A parapet four feet and six inch inclics high, and thick enough to resist musketry, will be, as a rule, all that is necessary. Ditches, as obstacles, will not be necessary. A sufficient obstruction can be obtained by scarping the slope, effected by cutting away its face until a steep slope is made. If a ditch is used, it will be placed ordinarily in rear of the parapet. If the high ground terminates in a plateau or large flat surface, it would be well to establish a line of strong field works, some two or three hundred yards in rear of the brow, which would command the works placed along the crest of the slope. 160. General plan. — The defence of a position is made in accordance with some general plan, which plan decides as to the predominance of the offensive or defensive features of the position, the points to be fortified, the number of troops that can be spared to occupy the fortifications, and the time in which the works must bo built. After these general points are fixed, the engineer may then consider the kind of works which will 193 FIELD FORTIFICATIONS. be best fitted for the purposes intended, and arrange the profile, defilade, and construction according to the natural features of the site and the means at his disposal to build the works. The position should be well selected, since no amount of skill can remedy de- fects which are fatal. The faults may be ameliorated, but the position can not be made a strong one if inherently weak. The engineer shows his skill in adapting his constructions to those positions which in themselves have some merit. This adaptation re- quires a thorough knowledge of the principles of for- tification and of the details of the art. How extensive these modifications may be can be seen by the examples just given. Bridge-heads. 161. Definition. — The term, bridge-head, (French, Ute-de-pont) is applied to any field work or line of works which is built to defend a crossing of a river at a particular point, and to prevent its use by an enemy. These crossings are made by means of bridges, ferries, or fords. Of these, the bridges are the most re- liable for all stages of water, are the most convenient, and are the constructions which are usually guarded and defended with the greatest care. The principles governing their defence are equally applicable to the defence of fords or ferries. riBLD FORTIFICATIONS UFON IRREGULAR SITES. 193 162. Object. — The object to be attained by the use of a bridge-head is to protect the bridge from the destructive measures of an enemy, and to prevent its use by him. This preservation of the bridge may be for the purpose of keeping open the communication with the opposite bank, with the expectation of using it either by small parties or by large bodies of troops. When intended for the use only of small bodies of troops, and to keep possession of the line of com- munication, the bridge-head used for the defence of the bridge, may be a simple field work, generally a redan, or a lunette. (Fig. 77.) Fig. 77. This half-closed work, a redan or a lunette, L ^s usnally placed on the road leading to the bridge and upon the side of the river towards the enemy. 194 fieTjD fortifications. Its rear should be closed by a stockade, and thii stockade connected with the approaches to the bridge, or banks of the stream, by a palisade. A temporary road, as shown by the dotted lines, should be built to allow of a free passage around the bridge-head, in- stead of going through it. If there is no danger of the enemy appearing in force and with artillery, a olock-house may be built within the bridge-head. The faces and flanks of the bridge-head should be placed so as to intercept all fire that the enemy can bring upon the bridge from any position which he might occupy. If there is danger of the enemy coming in force, either to destroy the bridge or to cross the river at this point, the bridge-head should be made strong enough to resist an artillery fire, and should be strengthened by batteries placed in positions like B? B B, by means of which a concentrated fire can be haa upon the ground in front of the bridge-head, and a fire into the interior of the work, as indicated upon the figure. Bridge-heads of this kind should be made strong enough to hold the enemy in check until re-enforce- ments could arrive, or until the bridge could be made useless to the enemy. Bridge-heads of this simple kind are useful t^" protect bridges which are or might be useful for recor noitering parties composed of small numbers. FIELD FORTIFICATIONS UPON IRREGULAR SITES. Vdl 163. Strong bridge-heads.— A bridge-head used to guurd a crossing when the army may move in force, either to make an advance movement, or to retreat, must be made strong, if the enemy is, or is expected to be, within its immediate neighborliood. The presence of bridges already constructed may determine the selection of the point at which a gen- eral may desire to cross his army, either in making an advance towards the enemy, or retreating from him. If he has to have bridges built, he will have some latitude in the selection of the point at which he may cross. The selection of this point will be governed by strategical, tactical, and technical considerations. The tactical requirements will be best satisfied, as a rule, when the point of crossing lies in a bend, with its convexity towards the army which is to cross. The bend will in many cases allow the establish- ment of batteries from which flanking and cross fires can be had upon the ground in front of the bridge head (Fig. 77). The banks in a bend are usually unequal in heiglit, the higher bank being on the convex side and op- posite to the bridge-head. This difference of heigh', will frequently allow the site of the bridge-head to b£ commanded by the fire from the opposite bank. There are other prominent tactical advantages ac companying a bend, sucli as tlie approaches to tlif 196 riELD FORTIFJCA TlOyS. river on the convex bank being concealed from view of those on the opposite side ; a greater protection being obtained for the bridge structure ; etc. Great as these advantages are, a good crossing re- quires in addition, if it is to be used by troops mov- ing in force, a roomy space in front of the bridge-head hidden from the view and sheltered from the fire of the enemy in which the masses can easily deploy. (Fig. 78.) In this example, the shelter is given by a line with intervals, L. L> L phiced some distance in front of the KIELD FO R TIFICA TtONS UP OX IE REG ULA H SITES. 10"? bridge-head and encircling the space to be occupied by the troops in their movements forward from this point, or in retreat. The line should be constructed in ac- ^jordance with the principles already laid down ; the works, in defensive relation with each other, and, where necessary supplemented by secondary works. The extremities of the line should, as in this case, rest upon the river or upon points which prevent its being turned. A bend in the river enclosing a narrow strip of land would be exactly fitted for a bridge, and for its bridge-head, but would not be so favorable for the passage of large bodies of troops, because of the limited space afforded to the masses for deployment after having crossed the river. There is shown only one bridge in the figure. In case of a rapid advance being made, or a quick retreat closely followed by the enemy, several bridges should be used. 164. Horn -works, etc. — The traces used for bridge-heads are of various kinds, and are modified by the irregularities of the site, by the directions of the approaches, and by the forms of the banks. Re- dans, lunettes, bastioned lines, cremaill6re lines, etc, of various shapes and forms, are used. Certain arrangements of the bastioned lines give rise to the bridge-heads known as horn vrorks, and cro-wn "works. 198 flELt) FORTtFICAtlom. Let a [)oinfc be taken in front of the bridge and some distance from it. Tlirough this point let a right line be drawn perpendicular to the general direction of the bridge. On this line thus drawn as an exterior side, let a bastioned front be constructed, and its sali- ents be joined with the banks of tlie river by straight lines, which are so directed that they can be swept by a fire from the opposite bank. The resulting trace is known as a horn-work. It is plain that this work will only be used when the main approach to the bridge is in the prolongation of its length. If through a point assumed in front of the bridge two right lines were drawn making an angle with each other, and prolonged until they reached the banks of the river, and on these two lines, as exterior sides, oastioned fronts were constructed, the resulting trace would be that of a crown-work. It is plain that this trace will be employed when the approaches to the bridge are oblii^ue to the di- rection of the bridge, and that the enemy would use one as quickly as the other. If there are several approaches, and the entire front is exposed to attack, a continued bastioned line might be used, enclosing the space in front of the bridge from bank to bank. In this case, if a salient occupies the central position, the line is known as a crown-work. In the latter case it is called a P'tPA.n FOiniFlCATtONS UPON JRliEGrLAk SITES. 190 complex crown- work, to distinguish it from one con- structed on two sides only, which is called a simple crown-work. When the bridge crosses the river at a point where there is no bend, it is frequently the case that works are constructed at both extremities of the bridge. When this is the case, the works form what :s known as a double bridge-head, to distinguish it from those just described, which are termed single bridge-heads. CHAPTER XIV. HASTY INTRENCHMENTS. 165. Hasty defences. — Hasty defences include all extemporized shelters, which are quickly con- structed (in a few hours at most) from materials found upon the spot where the shelter is needed. Ill consequence of the effectiveness of modern fire- arms, a body of troops can not retain a close forma tion for a single hour even, if in the presence, and exposed to the fire of an enemy in force. The men are forced to seek shelter, by lying down on the ground, or by crouching behind any slight inequality which may exist in the surface, or behind some kind of a screen which they may be able to construct. The screen may be two or three logs rolled together (Frontis- piece), a heap of fence rails, a slight mound of earth, or anything whatever its nature which will hide the soldier from the enemy's view. 166. Shelter-trenches. — The simplest form of shelter, for a soldier in open country, under ordinary circumstances, is a shallow trench, which will furnish from the excavation, sufficient earth when heaped upon the side towards the enemy, to screen the soldier in the trench from the enemy's view. Trenches of HASTY INTRENCHMENTS. 201 this kind are known as shelter-trenches, but are most frequently culled by the soldier, rifle-pits. Be- cause of the shortness of time i*equired to build these defences, they are known under the general name of hasty intrenchments. Hasty intrenchments were much used by both of the contending armies in the late war in the United States. They were used so frequently, and found so efficacious, that the men acquired the habit of in- trenching their line immediately upon halting after a day's march, if the enemy was near. No compulsion, no orders, even, were necessary for the men to begin this work ; the main difficulty was to make them de- lay long enough to allow a proper trace to be marked, by which they might be guided in the construction of their line. Instances are known, where the men, not having intrenching tools, executed the trench with the bayonet and tin cup. These shelter-trenches, thus rudely constructed, were deepened and strength- ened until they were able to resist field artillery, if the position was to be occupied for any length of time. Slight as these defences were during the early stage? of their construction, they formed, when de- fended by good troops, an obstacle difficult to over- come ; and they were captured only by extraordinary effort, accompanied by a great loss of life on the part of the attacking forces. 167. Construction. — The trencli represented in 202 :^IELto FORTtftCATtONS. Fig. 79 is the sniiillcst that can be made, upon level ground, and afford shelter to a man. Its depth in rear is one foot ; its width is five feet ; and its length is dependent upon the number of men using it. The earth taken from the trench and heaped npon the ground in front will make a mound fifteen inches high and at least two feet thick, aifording a screen from the enemy's view, and a tolerable shelter against musketry fire. A log laid in front of the trench and the earth thrown over and against it, adds materially to the protection afforded by the shelter. A trench of this kind can be executed by the soldier with a shovel in thirty minutes. 168. In one hour's time, a soldier can deepen and strengthen this trench, giving to it a depth of eighteen inches throughout its width of five feet, and Taising the mound to a height of eighteen inches. (Fig. SO). This additional depth of trench and height of mound allow the soldier using it, to take a kneeling position when firing ; a position more convenient than ihe reclining one necessary in the last case. 169. In from two to three hours' time, if the soil is not difficult to dig, the trench can be enlarged to a width of eight feet, and the earth excavated thrown upon the mound, raising it to a height of three feet with a thickness of about four feet. (Fig. 81). A trench of these dimensions allows the soldier BASTT INinENCmiENTS. 203 to occupy a standing position when firing and it ap- proximates to the form of trench to be constructed finally, if there is time for the purpose, and the posi- lior. is to be held. Fig. 79. mmm^mmm 170. It will be observed that the first trench offers no serious obstruction to the advance either of artillery or cavalry. In the latter forms, the trenches will obstruct, more or less, forward move- ments, if the trenches form a continuous line. At short distances, intervals should be left in the line of the trench, or ramps be arranged in the trench, so as to allow the artillery, etc., to march straight over the intrenched line if a forward movement is to be made. 171. Covered Communications. — Shel ter 204 FIELD FORTIFICATIONS, trenches are much used to afford covered com- munications, along a given front ; to connect the works in a '' line with intervals ; " to bring a mus- ketry fire upon ground which can not be swept by the fire from a particular work ; etc. The least dimensions of a shelter trench, when used as a communication for infantry only, are given in Fig. 82. Fig. 82. The trench should be made three feet deep, and four feet wide at the bottom. The earth should be thrown on the side towards the enemy, and then levelled off in the form of the superior slope of a parapet, so that the men in the trench can fire over this mass of earth. If the trench is to be used for the passage of ar- tillery, or to be used by bodies of troops passing from Fig. 83. one point to another along the front, its dimensions ehould be those shown in Fig. 83. HASTY lyTli Ey CEMENTS. 205 The least width at bottom should be made eight feet, and the height of the top of the mound of earth should be at least six feet and a half above the bottom of the trench. The side of the trench toward the enemy should be cut into off-sets, and arranged so as to allow a fire of musketry over the parapet. 172. Trace of shelter trenches.— Shelter trenches are rarely made to follow a straight line, but usually conform to the contour of the ground. The trace should be marked on the ground if there is time to do it. It will economize the labor of the troops, and avoid an unneccessary waste of time. The trace should be governed by the general rules laid down for field works, and great care should be taken that it can not be enfiladed by a fire of the enemy. Profiles are not necessary. The points which would be occupied by them may be marked by men standing upon the edge of the proposed trench towards the enemy. A line would then be marked on the ground, by a pick, passing through the points selected. Parallel to this line and twelve or fifteen feet in rear of it, the line of troops should be formed. The front rank, furnished with intrenchino; tools, would begin the digging ; the rear rank would lie down. Reliefs should be formed, and the trench lapidly executed. 206 FIELD FORTIFICATIONS. 173. Shelters for artillery.— The shelters for artillery or cavalry may be made in a very short time, in a way similar to that shown for the shelter trench for infantry. On nndulating ground, the shelter trench for in- fantry is frequently on the slope; the shelter for ar- tillery would generally be on, or behind, the crest. (Figs. 84 and 85). Fig. 84. It is seen that a piece of artillery on the crest of undulating ground can be quickly run under cover, :f it be desirable. (Fig. 84). This cover can be quickly and easily improved, by making a slight excavation and arranging a mass of earth in front of the gun. (Fig. 85). Fig. 85, Slopes of this kind could be used for infantry as well as for artillery ; and where a simple screen is the main object to be had, the communication would be along the reverse slope. HASTY INTRENCHMENTS. 207 174. Defence of a house. — It frequently hap- pens that the front of a defensive position is occu- pied by one or more houses. The houses may bo on the line of battle, or they may be close to it. If they can be readily put in a state of defence, they are so arranged ; if not, they should be torn down. A house, solidly built of stone or brick, and which will not be exposed to artillery fire, can be readily made into a defence of considerable strength. Brick houses with slate roofs are the best for the purpose. The first thing to be attended to, in putting a house into a defensive condition, is to clear the space in front of it of every thing which would screen an enemy's approach. The next is to loop-hole the walls, making at least two tiers of fire in the lower story. The loop-holes in the upper stories should be arranged to get a fire as close as possible to the foot of the walls of the building. Ditches cut near the walls, and the earth thrown against them, are recommended to keep the enemy off and to add to the resistance of the walls. All doors, sashes of windows, etc., inflammable in their nature, should as far as possible be removed and the openings barricaded by sand-bags, or by boxes and barrels filled with earth. Barrels filled with water should be placed in each room to put out any fire which might happen to 208 FIELD FORTIFICATIONS. break out in them. Earth in a moist state might be spread upon the floors, or on any of the flat surfaces, liable to take fire, to prevent their burning. 175. Tambours. — A house which has projecting parts, like bay windows, can be readily arranged to give a cross fire in front of its salient angles. The same result may be obtained where two houses flank each other. Where flank defence can not be thus obtained, it may be liad by using constructions known as tambours. (Figs. 86 and 87.) Fig. 86. A tambour is a stockade, generally redan-shaped in plan, placed in front of a door or other opening, in the wall to be flanked, and arranged with loop-lioles II A STY INTRENCUMENTS. 201-. to bring a fire upon the ground in front of the salients of the line to be defended. Fig. 87 It may have one or more tiers of fire ; it may be one story high, or it may be two. 176. Machicoulis galleries.— Where tambours can not be employed, a flank defence may be obtained by means of a construction known as the machi- coulis gallery. (Fig. 88). The object of this con- struction is to bring a fire upon the ground along the foot of a wall. Where balconies exist, galleries of this kind can be made from them. If no balcony belongs to the house, the gallery may be formed by breaking two or more holes through the wall, on a level with the floor of the eecond storv. 310 FIELU FORTIFICA TIONS. Througli the holes thus made, stout beams are passed and their inner ends firmly secured to the joists of the floor. These beams form the joists of the gallery, P'g«83. ^ Fig. 89. .ind they may be braced, or not, by diagonal pieces as shown in Fig. 88. On the outer extremities of these 'oists, a musket-proof shelter is constructed ; this shel- ter may be made of two thicknesses of plank, enclosing some light resisting material. The men fire through the bottom of the gal- leries upon the ground beneath, using pistols, if they have them, in preference to the musket or the rifle. 177. Expedients similar in principle to the machi- coulis gallery are used to bring a fire upon the ground in front of the wall of a house where the gallery can not be used. One of these is shown in Fig. 89, where the sash of tlie window is removed and a musket proof HASTY INTRENCUMENTS. 211 shelter is inserted in the window at an angle so as ip leave an opening at the bottom through which a man can fire his pistol. 178. Stone walls, hedges, etc. — Obstructions like stone walls, licdges, etc, can be utilized in the de- fence of a position. High stone walls can be loop-holed, and ar- ranged for defence in a way similar to that adopted in stockades. Low stone walls may have shallow trenches dug behind them, and the earth thrown over and against the wall. The top of the wall can then be crenelated or arranged with improvised loop-holes. Hedges can be made into serious obstacles to an enemy's progress, and can be quickly converted, by means of earth thrown against them, into good shel- ters, for the defence. 179. Woods. — If the woods are too far to the front to allow of their being defended, they should be cut down, or •' slashed," thus forming an obstacle, but not a screen to the enemy. If they are to be defended, a line of shelter trenches may be constructed along the outer edge of the woods, on the side towards the enemy, but within the edge 80 as to be concealed from his view. A thin strip of the timber may be cut down to form an abatis in front of the intrenchments. Good roads and open spaces should be arranged in 212 FIELD FORTIFICATIONS. HASTY INTRENCHMENT8. 213 the woods to allow, the troops and guns to move about freely. 180. Roads. — The roads in rear of a position should be arranged so that they can be freely used by the defence. Those in front, should be obstructed, and in every way made useless to the enemy. Bridges to be used by the defence should be put in good order ; those which would be of service to the enemy, should be destroyed. 181. Intrenched fields of battle. — All hasty intrenchments and defences have their primary use in defending a position which is to become, in a short space of time, the scene of battle. The principles already named, as necessary to be observed in fortifying a position for defence, are equally applicable to the methods used to intrench a line on the field of battle. Tiie more completely the general conditions for fortifications are fulfilled, the stronger will be the intrenched line. The example shown in Fig. 90 may be taken as an illustration of what is meant by an intrenched line of battle. An army corps is supposed to be marching on the road from A to B (Fig. 90), and the commander is instructed, if the enemy appears in force, to take a position and hold him in check. The position selected may be one which is to be held only for a short period, so as to allow time for ilie 214 FIELD FORTIFICATIONS. army to concentrate, or to force the enemy to concen- trate his forces to attack ; or it may be one in which a decisive battle is to be fought. It is supposed that the commander of the army corps, when in the vicinity of A> learns of the approach towards B of the enemy in force, and he feels obliged to hurriedly occupy a position to resist the enemy's advance. A reconnoissance has shown him, that the best natural position for him to occupy for this pur- pose, is the ground between L and R, and that the distance between thesS points is about four thousand yards. He finds tliat the high ground runs nearly perpen- dicular to his line of retreat through A? and is bounded upon the right by an unfordable stream with low and marshy banks. He finds the ground higher where the main road from A to B crosses it, than at any other point and that it slopes gently both to the front and rear, commanding all ground within cannon range. The natural features of the ground are shown in the sketch. — A wood W of considerable size in front of the centre of the left of the position ; a village at B) outside of cannon range ; a wood at D, also outside of cannon range ; a small collection of build- ings at the cross-roads at C| a wooded slope upon the right of the position ; a farm-house, capable of being put in a defensive condition, almost on the general litio HASTY IXTRENCUMENTS. 215 to be occupied ; etc^ form the most prominent features of the ground. Good roads traverse the country ; the one leading from A to B> being a turnpike; the others, common roads such as are seen generally in tlie country. The road from H to K has worn into the surface, forming a sunken, or what is generally known as a hollow road. The country is opju and cultivated ; inter- sected with ordinary farm fences, and divided into fields for pasturage, cultivation, etc. The commander ascertains from the reconnoissanco that the high ground at P must be held at all hazards; that this part of the line forms the key point of the position. That the woods at W are too large to be "slashed" in the time at his disposal. They must therefore be defended and made so strong as to re- move all danger of the enemy's obtaining possession of them. The number of troops forming the army corps is about thirty-four thousand, and is sub-divided into divisions, brigades, etc. For simplicity of details, the corps will be supposed, in this example, to be divided into six brigades, of six regiments each, and its com position to be as follows : Six brigades of infantry, three regiments of cavalry, twelve batteries of field artillery, three batteries of horse-artillery, and six companies of engineer troops. These will number about as follows ; . 27,000 1 men . 3,600 a . 2,250 a 900 a 1. 33,750 a 216 FIELD FORTIFICATIONS. Thirty-six regiments of infantry, . . . Three regiments of cavalry, . . . Fifteen batteries of artillery, . . . Six companies of engineer troops, . . Total Since the left of the line is to be the most strongly defended, the commander distributes his infantry as follows : one brigade upon the right ; two brigades upon the left ; two brigades, to defend the ground be- tween the right and left, and the remaining brigade in reserve. The brigades are placed about as follows : The first brigade being expected to defend the high ground at R, may be posted as follows : two regiments in the intrenchments s, s, on the right and left of the bat- tery at 1 ; one regiment in the skirt of the woods on the right, supported by another regiment near it but hidden by the slope of the hill ; two regiments should be placed a short distance in rear of the right, to form a second line and act as supports to the first line. The second brigade is placed on the left of the first, as follows : three of its regiments deployed be- hind the crest of the hill ; the remaining three, the troops being in column, should be in rear of the others, forming a second line. The third brigade is placed on the left of the second, as follows : three of its regiments deployed along the slope upon the HASTY INTRENCHMENTS. 217 same general line formed by the second brigade; one regiment placed in the works s, s, near the battery at 2, as a support to the guns ; the remaining two regiments in rear, forming a second line. The fourth and fifth brigades are to defend the left. The whole of the fourth brigade is posted in the woods ^ three of its regiments deployed along the skirts of the woods on the sides towards the enemy, and the remaining three, in column, in convenient supporting distances. The fifth brigade has one regiment posted at the farmhouse ; one, in the intrenchments s, 8, support- ing the batteries at 3 ; one, supporting the battery at 4 ; and the remaining three regiments behind the woods, as shown in the figure. The sixth brigade forms a reserve, and occupies a convenient position in rear near the road coming from Ai The artillery is posted as follows : two batteries upon the high ground on the right, commanding the approaches in that direction, — marked on the figure at 1 ; one battery, in the interval between the second and third brigades, at 2 ; two batteries, on the left of the third brigade at 3 ; one battery on the left at 4 ; one battery, in the front edge of the wood W ; two batteries, in the second line behind the second brigade ; one battery in the second line behind the third brigade ; one battery of horse artil- lery with the cavalry on the right ; and the remaining batteries, four in all, with the reserve. iJ18 FIELD FORTIFICATIONS. The cavalry is posted as follows : one regiment behind the right wing ; one behind the left wing ; and one with the reserve. The cavalry on the right may be divided into two parts, one part is posted behind the extreme right, the other behind the sec- ond brigade. The cavalry behind the left wing may be divided into two parts, one is posted behind and near the farmhouse, the other behind the wood W and near the main road. The engineer troops are distributed along the en- tire front, engaged in the preliminaries belonging to the work of intrenching the position. These prelimi- nary operations consist in putting the woods W ^^ front, the woods upon, the right, and the farm-house, in a condition of defence ; in laying out the shelter trenches,, a, b, c, etc., and posting the working parties detailed to construct them ; and in tracing the epaul- ments, 1. 2. 3 and 4, and the field works, s, s, s, etc., intended for the batteries and their supports. The woods W are put in a state of defence as de- scribed in article 179. The battery posted in the edge is placed so as to sweep with its fire both of the roads leading to the woods. The batteries at '6 and 4 are placed so as to cross their fire upon the ground in front of the woods. The woods on the right are arranged in a similai manner to afl'ord an obstacle to the enemy moving in thq,t direction, HASTY INTRENGHMENTS. 219 All obstructions existing upon the ground in front of the position which can be made to interfere with the movements of the enemy are preserved and added to ; all those which would screen his movements or afford him shelter, are removed or torn down. The hollow road in rear should have its banks cut down so as not to interfere with the movements of the troops in crossing from one side to the other. The farm-house is arranged as described in article 174. The shelter trenches and the epaulmonts for the batteries are constructed as previously described. The field works, s, s, s, etc., have the traces of blunted redans, or of lunettes with obtuse salients. The faces should be made from seventy to eighty yards long, and so directed as to sweep the slopes in front. It is recommended to place in each work, a re- dan-shaped, defensive traverse arranged as indicated in the figure, and to give it a command equal to or greater than the jiarapet of the work. These works should be placed, so as to leave an interval of not less than one hundred yards, between their flanks and the batteries they support. The jirofile of these works, 5, s, etc., should ap- proximate as nearly as j^ossible to the normal profile. The time disposable for their construction will de- cide upon the command they shall have. By placing working parties upon both sides of the parapet, con- structing the part in rear of the interior crest with iJ20 FIELD FORTIFICATIONS. the profile shown in Figs. 81 and 82, and construct- ing the part in front with the normal profile, a strong work can be quickly built. It will be seen, that the commanding general, by (.he use of these intrench ments thus hastily constructed, jan concentrate, without risk, the greater part of his corps upon the left, the key point of his position. By their use, he is able to supply the want of numbers and thus strengthen his line. The principles observed in this example apply equally to a position of greater or less extent. CHAPTER XV. ATTACK AND DEFENCE OF FIELD FORTIFICATIONS. 182. Attack. — An attack made to capture a field work, or to carry a line, may be a quick, sud- den effort, or it may be a prolonged one. The for- mer is known as an assault* the latter, a siege. Assaults are of two kinds ; open assaults and those made with great secrecy. Whatever be the kind of assault, it should be pre- ceded by reconnoissances, made as full as possible, for the purpose of ascertaining the best and easiest approaches to the work, the nature and position of the obstacles, the numbers and kinds of troops com- posing the garrison, and the strength and positions of the reserves exterior to the work, but near enough to take part in its defence. Particular attention should also be paid to the po- sitions for the artillery of the attack. These posi- tions should be such that the guns can bring enfilading fires, on the principal faces of the work, strong cross fires upon the point of attack, and if possible, a sweeping fire on the approaches to the work in rear. An important point, to be observed in this matter, ip )322 FIfJLD FORTIFICATIONS. to select, if possible, positions from which tlie guns will not have to be removed during the attack. 183. Open assault.— An assault of this class is made suddenly, and if possible, without the enemy's knowing it until the rush is made. It is an open assault, because the approach is made openly, though the attack be unexpected. The assault may be divided into three periods, as follows : 1. The preliminary operations, and the artillery attack. 2. The advance of the attacking troops from the cessation of the cannonade, until they arrive at the counterscarp of the ditch. 3. The assault of the parapet, and the capture of the work. 184. The preliminary operations consist in se- lecting the points of attack ; designating the troops to make the attack, and placing them in position ; organizing bodies of pioneers, or engineer troops to remove obstacles; etc. It is supposed that the out- posts of the enemy are all driven in, and his troops are within the work, or under its immediate protec- tion. A heavy, converging fire of artillery, within accu. rate range, has been deemed by military authorities to be an essential element of success in an open as- sault. The manuals all prescribe, in an open assault, ATTACK A yi) DKfKNCJB OP FIELD FORTIFICATIONS. 233 an artillery attack precediug the forward movement of the attacking forces. The objects expected to be attained, by this artil- lery fire, are as follows : 1. To silence the artillery of the defence. 2. To damage, and possibly to destroy the worka. 3. To inflict losses, and to harass the men to such an extent, that they may become demoralized. The first object is attainable. The theory of an open assault supposes a superiority of numbers upon the part of the attacking forces, and a superiority of artillery. When this is tlie case, the silencing of the artillery of the defence becomes a mere matter of time. The second object is hardly practicable so far as its destruction is concerned, if the work is made of earth, and has a good profile. The fire may, however, be successful in tearing up the superior slope, destroy- ing the improvised loop-holes, and producing irregulari- ties in the interior crest of the parapet. The amount of damage which can be made, will depend upon the amount of fire that can be concentrated upon the work. The effect of the damage will be to impair, more or less, the confidence of the men, in the efficacy of the work to shelter them. The third object may, or may not be, attained. The kind of shelters, the strength of the parapet, and the character of the troops are factors of the problem. The conclusions drawn from recent experiences 224 FIELD FORTIFICATIONS. are, that an attack by artillery, preliminary to an open assault, may be useful against works of weak profiles not provided with bomb-proofs, but against strong works with good interior arrangements, the artillery attack is of little service, and may be injurious to the attack so far as it serves the defence as a warning of the pi'oposed assault. 185. The features of the second period are the advance of the attacking forces, and their progress to the ditch. The advance is made by a line of infantry, in extended order, preceded by skirmishers. A second, and even a third line, follows the first. The skirmishers advance by ** rushes," taking ad- vantage of every inequality in the surface to shelter themselves, and being reinforced continually from the line in their rear. Accompanying the line of supports are pioneers, or engineer troops, provided with tools, etc, to remove or to make passages through all ob- stacles in the way of the approach. The skirmishers keep up a sharp fire upon the parapet, while the obstacles are being removed, and while the supports are forming in the ditch, or on the berm. The methods of removing these obstacles are de- scribed in the books, and manuals, relating to practical engineering operations. 186. The third period includes the assault upon the parapet and all succeeding operations. As soon as a sufficient number of the troops have ATTACK AND DEFENCE OF FIELD FORTIFICATIONS. 225 assembled in the ditch, or iii»on tlic berni of the work, the assault upon the parapet is made by a rush of these troops in a body up the exterior slope and over the the parapet into the work. If a defence is still continued, a hand-to-hand con- flict follows. This should result in a capture of the work in consequence of the superiority of numbers on the part of the assailants. If the defenders should have left the parapet upon the assailants reaching the ditch, it niay be that they have done so for the purpose of occupying some in- terior retrenchment, or block-house. In this case, the guns of the work, if any, should be turned against the retreating forces and steps be taken to attack the inner defences. An examination of the powder magazines should be made immediately, to guard against explo- sions either accidental or intentional. Steps should be taken, as soon as the work is cap- tured, to put it in a state of defence against recap- ture, if it is to be occupied, or to destroy it, if it is to be abandoned. 187. An attack made secretly. — The pre- liminaries of a secret attack arfe identical with those of an open assault, except in the secrecy of the move- ments which are made. The troops, who are to make the attack, are kept in ignorance of the object of their movements until they are assembled at the point from which they are to make the attack. 22 G PTELD FORTlPtCATtOX!^, The succoss of the attack depends upon its being unexpected by the defence, and upon finding the defenders, in a measure, unprepared to resist it. An attack of this kind is a surprise, and is known fre- quently as an attack by surprise. 188. Success in either case, an attack made openly or secretly, is greatly dependent upon finding the enemy unprepared to resist the assault. The attack should therefore be made suddenly, and without the enemy suspecting the intention. 189. Attack by artillery only. — Under the circumstance that the attacking forces are overwhelm- ingly superior in artillery, it is possible, in works hastily constructed and not properly provided with interior defences, to force a garrison to capitulate by use of artillery alone, supplemented by sharp-shooters who may be able to get near enough to the works to pick off the gunners of the work. 190. Key-point. — Great care must be taken to direct the assault upon the key-point of the in- trenched line or position. The mistake has been fre- quently make of directing the main attack upon the wrong point, which attack when successful, produced no lasting benefit to the victorious troops. 191. Defence. — Fortifications are inert masses, passive in their nature, which become obstacles to an enemy's approach only when fully manned by well- armed, courageous and vigilant troops. ATTACK AND DEFENCE OF FIELD FORTIFICATIONS. 22? A field work performs all that is required of it, when it compels a force, superior in number to its garrison, to resort to the tedious and costly operatibns of a siege to take it ; or makes this force move in an- other direction to gain its ends. The military man siiould remember that a well- arranged intrcncliinent, or field work, should obstruct the enemy's approach, as well as give shelter to its de- fenders. It is a question, oftentimes, difficult to an- swer, as to which of the two, the shelter to the defence, or the obstruction of the enemy, is the more import- ant. Either one gives an increase of resistance to the defence, and the two combined form an obstacle diffi- cult to surmount. An enemy may succeed by brute force and sheer numbers, in carrying a position in which the defend- ers are sheltered simply, or a position arranged with obstacles that hold him under fire at close range ; but the success is accompanied by such heavy losses, that the victory, in many cases, had better be classed ii8 a defeat. This is particularly so, where the two are combined. It is well to remember, that a vigorous defence requires that every part of a field work, or line, -should be guarded by a sufficient number of troops to repel any assault that an enemy might make, and that numbers alone are not sufficient. Endurance, cour- age, and vigilance are necessary in the commander J^28 FIELD FORTinCATIONS. and his troops. N"o better motto can be devised for the banner of the defence, than the trite aphorism so frequently cuoted, " Eternal vigilance is the price of liberty." The memoranda, compiled for the use of the offi- cers, commanding the different field works, surrounding the City of Washington, during the war of 1861-5, may be quoted in this connection. The precautions to be taken, and the course advised for each commanding officer, are given in detail and will be as applicable in the future as they have been in the past. 192. Memoranda compiled for the guid- ance and information of officers serving in the de- fences of Washington. "1. The number of men required for the garrison of each work (artillery and infantry supports in- cluded) has been calculated, and should be known to every commanding officer of a fort ; but it will be for the brigade and division commanders, or for the com- manding general, to determine the necessity of filling up each garrison to its full siege complement and of manning the connecting lines of rifle-trenches. *• 2. It is the duty of a commanding officer of a fort to see that he has all the means and appliances that may be wanted during a siege provided before- hand. If his position be somewhat isolated and where the enemy may cut off his communications, he \ ATTACK AND DEFENCE OF FIELD FORTIFICATIONS, 229 should see that he has an ample supply of provisions stored, either in his fort, or in a secure place in its rear ; and he must take measures to keep an abun- dant supply of water in the fort, both for the use of the garrison and to extinguish fires. '^ He should see that his fort is provided with all the tools that may be wanted during the siege, par- ticularly with shovels, picks, axes, saws, auger^?, and hammers. Further, he should provide, either bc- foreliand or as soon as possible after the siege com- mences, all the materials that may be wanted during the siege, such as an abundant supply of timber, of plank, of nails, spikes, of sand bags, gabions, and fascines. Timber is of the first importance ; a large supply of it should therefore be secured. *' 3. When any part of the line of defence is threat- ened with an attack, all houses, trees, bushes, and in general everything that could be used as a cover by the enemy's sharpshooters, should be at once removed for a distance of at least six hundred yards from the line. If sharpshooters can be concealed within that distance of a fort or battery, they will pick off the canuouiers and the guns cannot be served without serious loss. '* In conducting the defence of a fort, each company or detachment, should have its particular post and duty assigned to it, and receive special instructions as to their duties in all possible contingencies. 230 FIELD FORTIFICATIONS, " Commanding officers should be careful to see that all their subordinates thoroughly understand the ground, both in his front and rear, and are familiar with all the roads and paths; they should know by a careful examination of the ground in front the points which an enemy would select for batteries, and the exact ranges to such points, and should study out how best to arrange their own guns to contend with such batteries should they be erected ; they should know the relations of the fort they occupy to the adjacent works, and be familiar with all the re- sources that may be made available during the de- fence. This information should be imparted by them to the non-commissioned officers, and in some cases to the privates ; for it should be remembered that the fate of the fort may depend upon the good or bad conduct of one individual. " There should be a reserve in all cases. It should be posted in the bomb-proofs, or behind traverses and magazines, or under temporary shelter made by lean- ing timber against parapets, magazines and bomb- proofs, or by digging trenches in the ground and covering them with timber and enrth. " Strict vigilance should be exerted to guard against surprise ; for this purpose, when the enemy approaches, a chain of sentinels should be posted in front of the works, and as far to the front as practicable, taking particular care to have them posted at all points ATTACK AND DEFENCE OF FIELD FORTIFICATIONS, 231 where an enemy might approach the works witliout being seen, either for the purpose of reconnoitering or delivering an assault ; but the general arrangement of sentinels must be made by brigade or division corn- man ders. " By keeping up, both by day and night, such a chain of sentinels in front of the works, and by post- ing, where practicable, a few sharpshooters in holes or rifle-pits in front of each fort, the efforts of the enemy to obtain the information he wants before commencing the siege batteries will be greatly re- tarded, and time afforded to the defender for com- pleting the preparations to receive him. " If the sentinels are driven in and an attack is ap- prehended at niglit, fire and light balls should occa- sionally be fired to the front, taking care, however, not to set fire to the abatis ; and in this case the guns should be double-shotted, and pointed if possi- ble, by dayliglit. so as best to sweep the ground with- in a distance of six hundred yards, depending on the configuration of the surface in each particular case. "If the ditches are not flanked shells loaded with service charges and ten-second fuzes, should be placed on the banquettes in charge of men specially instructed how to use them. These men should be provided with pieces of burning slow-match. "Hand grenades, loaded and capped, should also be placed ready for use, and, finally, the men sliould sleep 1^32 FIELD FORTIFICATIONS. at their guns and every man should know his post and his duty. Shells and also hand grenades may be exploded by attaching a string in such a way that the act of rolling thorn or throwing them will, as soon as the string is brought taut, explode a percussion cap or friction tube. " 4. If the enemy should open his attack by a warm cannonade, and concentrate his fire upon a particular fort, the troops should not be unnecessarily exposed to it, if they can be sheltered near the posts they are to occupy when an assault is made. If the cannon- ade should become too warm for the garrison to reply without too much loss, the field and siege guns should be removed from their embrasures and placed behind the parapets or, in case of an enfilading fire, behind traverses, bomb-proofs, etc. " During a cannonade the dead and seriously wound- ed should be kept out of sight. '' When the assault is made, this cannonade of the enemy must cease in order not to injure his own troops. The guns are then run into position and every man resumes his post. Tlie men should be instructed to reserve their fire until the enemy has arrived at cer- tain points, marked out in front of the works, where it will be most effective. *' Particular attention should be given to securing the gate-wa}^ The abatis in front should be made con- tinuous. As the gates are made to open inward, the ATTACK A\n DEFENCE OF FIELD FORTIFICATIONS, 233 enemy cannot easily open them outward. The danger ko fear is that he will force them inward, or lift them off their hinges. To prevent the first roll a heavy log against the inside of the gate, plant posts or drive strong stakes behind it, and, for greater security, cover the log with earth. The second danger may be pre- vented by '* upsetting " the head of the pintle upon which the gate turns, or still easier by driving a strong spike immediately over it. If the gate- way is not flanked it may be necessary, in some cases, to increase the dirt over the log until it becomes an infantry para- pet. Communication with the interior may be kept up by one or more light movable bridges, made of plank or boards, leading from the parapet across the ditch. These bridges should be kept inside the fort at night, or when an attack is anticipated. " 5. Every precaution should be taken to secure the magazines against vertical or curved fire. The en- trance is the weak point ; hence, where necessary, cover it by additional earth ; put splinter-proof guards, made of timber or plank, around the door, and have water at hand, in barrels or casks, to extinguish any fire near the doorway ; keep both doors shut, especially the inner one, allowing only one or two men to be inside, and only the ordnance sergeant, witli an assistant, on the stairway to pass out the ammunition that may be called for. Keep all the ventilators closed, and fill as far as practicable, all ventilating ^34 FIELD FORTIFICATIONS. tubes with earth both outside and inside of the magazines. " See that an ample supply of wads is kept on hand outside of tlie magazines, and that the ordnance ser- geant makes careful report of the ammunition ex- pended, and that it is promptly replaced. "Fill at once any holes made in the magazine cover. Logs, fascines, or even sticks, laid against its exposed side, greatly reduce the penetration of shot, particu- larly of elongated projectiles, by deflecting them. "6. Build merlons between barbette guns, and par- tially till wide embrasures, as soon as the positions of the enemy's batteries and the proper direction of fire of each gun are ascertained. Cut away the foot of scarps to render escalade more difficult, taking care not to endanger the stability of the parapet. Use earth so obtained for making a glacis and traverses across the abatis, if it be threatened by an enfilading fire. A few piles of eartii across the abatis, particularly if the earth be wet, is a great security against such fire. ^* Commanding officers of the forts cannot be too strongly impressed with the fact that the abatis is one of the main sources of strength to a field work. It should be carefully protected from injury and dep- redation, fire, etc. " 7. Bury percussion shells or hand grenades, to act as torpedoes, in the bottom of the ditch and outside of the abatis. ATTACK AND DEFENCE OF FIELD FORTIFICATIONS. 235 "8. Put up traverses on all faces liable to be en- filaded, to protect the guns, even if to obtain room for them some of the guns have to be removed ; repair all damages to the parapet on the following night, if not practicable to do it before. "Earth may be obtained for the above uses by ex- cavating in the terreplein for bomb-proof shelters, and by digging pits or holes., about three feet wide and deep, where ricochet shells are most likely to fall, and where the excavations will not seriously interfere with the defence ; remove all sheds and wooden buildings lest they take fire. **9. Construct temporary banquettes on all bomb- proofs and magazines, to afford an infantry fire on the probable front or points of attack. These may be made with plank resting on trestles or posts, or by cutting away the earth so as to afford standing room for infantry, with a parapet in front. ** 10. When, or before, the enemy's approaches have been advanced to the vicinity of the work, a surprise, or sudden assault upon one or more of the forts, es- pecially by night, may be anticipated. Under such circumstances it is the duty of the commanding offi- cer, though he may have resorted to every expedient to retard the siege, to impress upon the garrison that they cannot, without loss of honor, either abandon the fort or surrender it without resisting at least one assault. Such is the inexorable law of war, and ii 236 FIELD FORTIFICATIONS. should not be forgotten that it holds good whether or not there are other works in the rear to which the garrison might retire with comparative safety. When the attack is about to be made, the command- ing officer of a fort should endeavor to inspire hia men with confidence in their powers of resistance, with self-reliance and enthusiasm. *' Every preparation should be made to resist the passage of the ditch, by the fire of artillery and in- fantry, by torpedoes, by loaded shell and hand-gre- nades, and by attacking the enemy on the parapet. *' The commanding officer must decide beforehand, according to the particular circumstances under which he is called upon to act, how this attack should be made. If there be, behind bomb-proofs and maga- zine, banquettes and breast-heights for infantry bear- ing on the points of attack, then, at the proper mo- ment, the field-pieces and howitzers should be with- drawn from the platforms, loaded with double charges of canister and placed in positions where they may still be used against the enemy as he appears on the crest of the parapet or descends to the terreplein ; while the infantry should line these interior parapets and aid, by their fire, in driving the enemy from the work, the reserve charging furiously with the bayonet on the appearance of any confusion or disorder. "If there be no such interior lines of infantry fire on the point of attack, then the enemy must be mot ATTACK AND DEFENCE OF FIELD FORTIFICATIONS. 237 with the bayonet on the top of the parapet. For this purpose one or more steps, made of boards, should be prepared beforehand, to enable the infantry to mount the parapet, and it should be impressed on the defenders that the assaulting troops, arrived at position, will be greatly fatigued, and necessarily in disorder, and, moreover, will be cut off from all ex- ternal support, and hence that it is an opportune moment by a vigorous assault with the bayonet to hurl the enemy into the ditch and to retrieve, with disaster to the foe, the endangered possession of the work and of the defensive line on which the safety of the nation depends. '• The firing of the guns, particularly howitzer and field-pieces, loaded with canister, should be continued as long as possible in order to delay the advance of the supporting columns ; and, if the assault fail and the support be driven back in disorder, a sortie in force may be made and the enemy pursued into his works. **Such sorties, however, should not be attempted by the troops in any particular fort, but should be made, under the direction of the commanding gen- eral, by the outside reserves, supported by the fire of all the forts and batteries bearing on the position to be taken." 193. Examples. — The most noted examples in re- oent wars, of inerenched positions, in which the work? 238 FIELD FORTIFICATIONS. used were field fortifications, and which illustrate the attack and defence, are found iu the cases of Sebastopol in 1854,Vicksburg and Port Hudson in 1863, Richmond and Petersburg in 1864, Plevna, in 1877-8, etc. The student is referred to these particular examples for the details which cannot be given here. From the many examples, reference will be made here only to the assault made upon Fort Sanders in 1863. After the battle of Ohickamauga, General Long- street was sent into East Tennessee to capture Knox- ville, then occupied by the United States forces com- manded by General Burnside. The latter succeeded in intrenching his position, and held the Confederate troops at bay. After the victory at Chattanooga, General Grant sent troops to relieve General Burnside, and General Longstreet knowing of their approach determined to risk an assault upon Burnside's position. He selected the key point of the position, as the point of attack, which was defended by an unfinished earth work, laid out under the direction of Captain Poe of the United States Engineers. There had not been time to make an abatis, to build powder magazines, or even to revet the work as it should have been. A wire entanglement had been arranged in front of the work, fastened to the stumps of the trees that had been cut down to clear away the ground. ATTACK AND I) K FENCE OF FIELD FORTIFICATIONS. 239 At dark, on the 2Sth of Nov. 18G3, tlie Confeder- ate sharp-shooters were pushed forward to within rifle range of the line. At half past six in the morning of the 29th. a heavy fire of artillery was opened on the fort and lasted for twenty minutes, or half an hour. This fire ceased, being replaced by a fire of mus- ketry, and a rush of a brigade of infantry upon the salient of the work. The wire entanglement tripped many of the assail- ants, who fell upon the ground, but numbers of them succeeded in entering the ditch and attempted to as- sault the parapet. The heavy fire of the defence from the flanks and from the parapet, drove them back and, the assault failed. General Longstreet then withdrew his forces. The readiness, with which the assault was met, prevented its success. 194. The history of attacks made upon field works in recent wars, shows that assaults upon them were generally unsuccessful, where a vigorous defence was made ; and when successful they were accompanied with great loss of life, and injury to the assailant. The conclusion may be fairly drawn that a field work, vigorously defended, cannot be captured by as- muU if it has a good profile and is j)rovi(led with the accessory means of defence ; or if it is commanded by other works which can cot be assaulted ; or, if the 240 FIELD FORTIFICATIOXS. garrison can be readily reinforced from a strong body of troops within supporting distance. A vigorous defence supposes vigilance and secu- rity against surprise. An attack may be, neverthe- less, an unexpected one, and may task all the efforts of the defence to repel it. The success of an assault is greatly dependent upon the suddenness with which it is made, and a commander should remember, " When 'tis done, then 'twere well It were done quickly." CHAPTER XVI. SIEGE WORKS. 195. Siege Operations.— Fortified positions, vigor- ously do onded, can rarely be taken by assault. As a rule, positions of this kind can be carried only by the attacking force getting near enough to the defenders to overpowei them by the actual contact of superior num- bers. The assailant, to profit by his numerical superiority, must remove or overcome the obstacles between him and the defence. To be able to remove these obstacles, he must protect himself as much as possible from the fire of the defence, so that he may get near enough to the obstacles to overcome them. This moving forward " under cover," and removing the obstacles in his way, make the advance of the assailant a slow one. The attack necessarily becomes a protracted one, and re- ceives the name of siege. (Art. 182.) The labors and movements of the assailant, by means of which he gets near enough to the defence, occupying a position of this kind, to render an assault practicable, are known as siege operations. 196. Investment. — The final success of a siege is greatly dependent upon the isolation of the defence 242 FIELD FORTIFICATIONS. from all aid. This isolation is effected by cutting the communications of the defence, and making it impos- sible for him to receive aid, either by reinforcements, or by supplies. This interruption of the communications and the isolation of the defence form the investment of the position. The investment is usually performed by a strong body of troops detached from the attacking force, which body moves quickly and suddenly, surrounding the po- sition, and seizing all the avenues of approach. A chain of outposts and sentinels, placed just out- side of the range of fire of the defence, but close enough to watch all the avenues leading to the position, is established by the investing force. This chain is drawn in nearer to the position at night, and moved back a short distance in daytime. The terms nightly cordon and daily cordon are frequently used to designate this chain. 197. Posting the attacking force. — The main body of the attacking force follows closely the investing detachment, and takes a position in supporting distance of the cordon. The question is then decided as to the character of the attack which is to be made, that is, whether an assault is to be tried, or whether a protracted attack is to be made. In the latter case, whether the attack shall be a simple blockade, a bombardment, or a regular sie|;e. SIEGE WORKS, 243 A blockade consists in surrounding a position and preventing supplies from entering the place, until the defenders, driven by want, are obliged to surrender. This is a slow process, but effectual in many cases, especially when the nature of the surrounding country enables the assailant to make the investment complete, and when the troops defending the position have scanty supplies. A bombardment consists in directing a heavy fire of shot and shell upon the position, destroying the defences, the magazines, supplies, etc., and wearing out the powers of endurance of the defence. A regular siege consists in approaching the position under cover, removing all obstacles in the way of an attack, and making an assault upon the defence. A complete or systematic siege of this kind is under- taken when the position cannot be carried by assault, and when the methods of blockade or bombardment are too slow or too uncertain. These latter methods are both used, however, in conjunction with the regular siege, if possible ; and, in the Ciises used to illustrate siege opera- tions, it is supposed that the investment is complete, and that the necessary arrangements lor a bombardment are made. A regular siege being determined upon, the general in command of the attacking forces designates the points exterior to the position which are to be occupied 244 FIELD FORTIFICATIONS. by the besieging forces. The camps are then laid out, being placed beyond the range of the heaviest guns of the defence, and, as far as practicable, upon sites favor- able to the health and comfort of the men. Good communications are at once established between all the camps, and all obstacles, which would impede the free circulation of the troops from one part to another of the ground occupied by them, are removed, or modi- fied. Every precaution should be taken to prepare the ground so that a rapid concentration of the troops can bo made, whenever it may be necessary. Strong field- works should be constructed in the vicinity of those camps exposed to attack. 198. Lines of countervallation, and of circum- vallation. —The line of field-works constructed in front of the camps,and on the side next to the besieged position, to defend the camps, parks, and trains against attacks which might be made by the besieged, is called a line of countervallation. Formerly, a line of works placed on the opposite side of the camps was used to prevent small detachments from slipping through the lines of the besiegers, and going to the aid of the besieged. This line was called a line of circumvallation. Such a line is not often used at the present day, because of the greater mobility of modern armies. A line of this kind might be employed as a line of defence against a reliev- ing army. Even in this case — the advance of a relieving army — it would be better not to wait passively within SIEGE WORKS. 245 the lines until attacked, probably by both the leliev- ing forces and the besieged at the same time, but to move out and meet the relieving forces at some other point. When these lines are used, circumstances will decide as to the kind, whether they shall be continuous lines, or lines with intervals, etc. The activity of the defence, the nature of the ground, the strength of the relieving army, etc., are all factors of the problem which is to be considered. The question of moving out to meet a relieving army involves, frequently, the question of raising the siege. If the besieging army is strong enough to permit it, a force is usually detached to watch the movements of the relieving army, while the main body remains prosecuting the siege operations. This detached body is known as an army of observation. 199. Preliminaries. — There are many things to be attended to and procured before the actual labors of the siege can be vigorously and systematically prose- cuted. If the siege is to be a vigorous one, these things must not be neglected. Principal among these, are the location of the parks and trains in convenient and secure places; the construction of magazines for am- munition, supplies, etc.; the supply of necessary tools and implements ; the supply of materials to be used in the siege works ; the preparation of these materials ; the complete reconnoissance of the fortifications defend 246 FIELD FORTIFICATIONS. ing the position and the groimd immediately in their front, etc. All these preliminaries should be attended to and provided for, if practicable, while the main body is arranging the camps and intrenching the ground to be occupied. 200. Selection of the side of attack. — The gen- eral, in selecting the side upon which to attack, tries to choose tliat portion which, being gained by him, forces the defenders to surrender, or to retreat, if there be any way for them to retire. That point wliose occupation compels an abandonment of the position, or its surren- der by the defence, is known as the key-point. Its selection proves the skill and fitness of the general for the command entrusted to him. Previous to an advance upon the position, the gen- eral has acquired more or less information about the defences, the strength of the defenders, the nature of the ground, etc. Upon reaching the spot, reconnois- sances are made to verify, or correct, this information, iind add to it in every way. These reconnoissances are supplemented by instru- mental surveys, and by other means, whose object is to determine the exact distances, and the true directions of the lines of the defence, the general features of the ground, and the kind of obstacles to be encountered. The general has to decide from this information thus gained, viz : StEGE WORKS. 247 1. Which part of the position is easiest to carry ; 2. Which part carried gives possession of the rest ; or, which part is the key -point ; 3. Which side of the part selected is the best on which to make his approaches ; and, 4. Which part selected would be the best, taking into consideration the establishment of his depots and lines of supply, and the probabilities of an attempt to relieve the besieged. These questions are partially answered before the posting of the besieging army is completed, as it would be bad policy to have the troops encamped too far from the ground where the main operations of the siege are to be conducted. 201. First parallel. — It does not fall within the scope of a work like this treatise to give the details con- nected with siege operations. These details are fully given in the Manuals prepared for Engineer troops, and for the Artillery. But as every officer of the army in actual service is liable, at times, to be employed more or less upon works of a similar nature, it is thought advisable to describe the simpler portions which may fall to his lot to execute. The portions of the defences which are to be carried having been selected, the general gives instructions to make preparations for beginning work upon the bat- teries and first parallel. The first parallel is a simple trench, such as 248 FIELD FORTIFICATIONS. described in Art. 171, and is inteiided us a protection for the infantry line which is drawn up nearly parallel to the side or front to be attacked, hence its name. It also affords a secure base for nearer approaches upon the position. Before the improvements in modern artillery, the construction of the first parallel and of the covered communications leading from it to the depots in the rear, formed the first steps, when breaking ground for a siege. As soon as it was dark, the working parties were posted just out of range of grape-shot, and were pro- tected by strong detachments of troops under arms. The work was immediately begun, and was practically finished by morning. Even if discovered by the besieged, no particular interruption was expected, because of the inaccuracy of the fire at night, and, the distance of the working parties from the works. Under the protection of the troops placed in this parallel, the batteries, if any were to be used at this period, were begun on the second night, and the approaches were pushed forward. It is usual now to establish batteries before, or simul- taneously with, the construction of the first parallel, so as to engage the artillery of the defence, and keep down its fire. Otherwise, the working parties would suffer great loss from the artillery fire of the besieged, and would not be able to finish their tasks by morning. Batteries armed with heavy guns are, in some cases, constructed at distances of from one to three miles from SIEGE WORKS. 240 the works to be attacked, and, at a specified time, these guns open their fire upon the besieged. Field batteries are also, under cover of darkness, pushed close in, and add their fire to the others. Skirmishers move in close to the position, covering themselves in rifle-pits and shelter- trenches, and keep up a warm fire on the be- sieged. All this is done before the commencement of the first parallel, and for the purpose of testing the endurance of the besieged. In the meanwhile, prepara- tions are made to begin work upon the first parallel, and if the bombardment fails to force a surrender, the work is begun upon the parallel. 202. Construction of first parallel.— The posi- tion of the parallel should be marked or traced upon the ground before the time of beginning the work, care being taken to have no marks which would at- tract the attention of the besieged. This tracing upon the ground is executed by engineer soldiers, and in accordance with the methods given in the manuals of engineering. The working parties, detailed from the infantry, are brought to a convenient spot near the place, and fur- nished witli picks and shovels. It is recommended that they be divided into reliefs. On the arrival of the engineer officer reporting the tracing completed, the working parties move forward to the ground to be occupied, being led by this officer. The first relief is marched off in fours, by file, or in '260 FIELD FORTIFtCATIONB, column, according to the circumstances of the case. Each man carries a pick and a shovel, and slings his piece so as not to interfere with his movements. Upon reaching the ground, the men are extended along the ]ine, indicated by the tapes, which marks the direction of the parallel. Engineer soldiers assist them in this ex- tension, and show the men their places and their tasks. The men are placed five feet apart. Each man drives his pick into the ground, on the left of his task, lays his shovel in front of him, behind the tape, unslings his musket, takes off his waist-belt, and gets ready for work. At the proper command, he begins to dig. The men are instructed to keep silence, and to allow no clashing of tools, or of arms. All words of command are to be given in a low voice ; lights and smoking are not allowed. Lanterns of a peculiar shape are allowed to the engineer soldiers, who use them in tracing the line marking the parallel. The men begin their work as soon as they are told. Each man digs into the ground, making a hole about three feet in circumference and four feet deep, throw- ing the earth in front of the tape, and at a distance from him equal to the length of his shovel, which is a little over three feet. He then widens the hole to the limits assigned him. Fig. 91 shows the profile of the parallel excavated by this method, known as the simj)le trench. SIEGE WORKS. 261 The space, A. sliows the portion which is to be exca- vated by the first relief, who, having executed the task assigned it, returns to camp, leaving the tools upon the Fig. 91. I B j C 10 1 ground. The second relief executes the part marked B. The third relief completes the part marked C, and con- structs the lower step, using fascines, as shown in the figure. In the days of smooth-bore pieces, this first parallel could be executed within GOO yards of the salients of a fortified place, this being outside of the range of grape- shot ; but in these times, and in the presence of an active enemy, well supplied with arms, the distance must be much greater. This distance will depend upon the features of the ground, which always afford more or less cover to the working parties, as well as upon the nature of the defence. The ground to be occupied by the first parallel may be obtained by pushing forward skirmishers, who in- trench themselves as they advance. Having gotten pos- session of the ground, instead of building the parallel in a single night, the parallel may be constructed by throw- 362 FIELD FORTTFICAflONS, ing up trenches to connect the rifle-pits and shelter trenches occupied by the skirmishers. In this case, a strong body of covering troops should be used to guard the workmen from the sorties of the besieged. The length of the parallel must be sufficient to em- brace the front of attack, and the parallel should have its extremities protected by strong field works. 203. Approaches.— The approaches, or boy- aux, as they are sometimes called, are trenches whose general direction is towards the works to be captured. They have a zigzag direction to prevent being swept by an enfilade fire, the prolongation of each branch passing outside of any work of the defence which is in range. The profile of an approach is shown in Fig. 92. Fig. 92. ^!5^ Each branch of an approach overlaps that behind it by about ten yards, to afford protection against enfilade, and to serve as a trench-depot for trench-materials. The dimensions may be the same as those used in the first parallel, and frequently these are adopted, if there is to be much use made of the approach. In the figure, the approach is made only nine feet wide at bottom. Its SIEGE WORKS. 253 construction, when it is a simple trench, conforms to that already described for the parallel, and can be under- stood from the figure. In both the parallel and the approach, it is recom- mended to slope the bottom of the trench to the rear, giving a fall of about six inches. This provides for drainage, and also affords greater protection to the men rising them. It will be observed that steps are not mdicated in the approach, like those shown in the parallel. They are used in the latter to allow the guards in the trenches to fire over the crest of the parapet. If it bo required to post troops in the approach, or have men fire from it, steps should also be placed in it. 204. Fljring trenchwork.— It is not always prac- ticable to use the simple trench in these constructions, in consequence of the exposure of the working parties to the enemy's fire. A more expeditious method of obtain- ing shelter is adopted. The method used is known as the flying sap, or better, flying trenchwork. Fig. 93. This method consists in placing a row of gabions along the front uf the tracing tape, and filling them as 254 FIELD FORTIFICATIONS. quickly as possible with the earth excavated from the trench. When the gabions are filled, the rest of the trench is excavated, and the earth thrown over and be- hind the gabions, thus forming a parapet. (Fig. 93.) By this method it is seen that shelter is obtained much more quickly than by the method of the simple trench. 205. Second and third parallels. — Tactical con- siderations require the construction of other parallels, as the besiegers approach nearer the work. There should be at least two more, making three in all, and it may be necessary to use a greater number. The French, at Sebastopol, found it necessary to use seven. The second should be near enough to the first to have the workmen constructing it under the protection of the troops posted in the first parallel. This second parallel is within close artillery range of the defences, and nearly within the zone of accurate fire of small arms. It must, therefore, be executed by the method of " fly- ing trenchwork." From this parallel the work has to be carried on by trained engineer soldiers. The third, or last, parallel is so close to the work that there is a prospect of the besiegers reaching the parapets of the defences by making a rush from the par- allel. This distance to be rushed over is usually assumed to be not more than about thirty yards. Assuming the position to be fortified by a bastioned SIEGE WORKS. 255 \Tork, F, (^ig. 94) with out-works, A, B, C, etc., the rela- tive positions of tlie parallels and the approaches can be seen in the figure. Fig. 94 \ \ \ Th}r