TABLE,

shewing the dimensions, in feet; and the superficial content of earth of banquettes, parapets, and ditches, of field works.

Place pickets in a line (in length conformable to the side of the intended work) at each end of which erect perpendiculars equal in length to the side first marked out, and join the termination of these lines; which will complete the perimeter of the redoubt.

Note. A perpendicular is raised on a given line, with a chain or cord, by forming a right angled triangle from the numbers 3, 4, and 5, or any multiples thereof, and extending the cord, &c., so that the base may correspond with the base line of the pickets, and the perpendicular be in the direction of the side required.Vide PRACTICAL GEOMETRY.

2. Pentagonal redoubt.

With a chain, tape, or cord, construct, and lay down with pickets five similar, and contiguous triangles, having their bases, which form the sides of the pentagon, in the proportion of 47

to the other two equal sides, the length of each of these being 40.

3. Hexagonal redoubt.

From a central point with a chain, or line, construct, and lay down with pickets six equilateral, and contiguous triangles, the bases of which will form the required hexagon.

4. Octagonal redoubt.

Construct a square (vide No. 1), from the centre of each side of which erect perpendiculars outwards, in length proportional to the side as 13 to 60 (nearly 1 to 5); join the extremities, or termination of the perpendiculars, to the angles of the square, which will determine the sides of the octagon.

Note 1.-The directions for the construction of the pentagonal, and hexagonal redoubts are on a small scale; but the redoubts may be increased by the equal extension of the interior sides of the triangles, until the bases are sufficiently long for the periphery of the work required.

Note 2.-By means of the pocket sextant, prismatic compass, or reconnoitring protractor, the pentagonal, hexagonal, and octagonal redoubt may be thus traced on the ground. From a central point place pickets at the requisite distance from each other, and in the direction of lines drawn from the angle of the centre of the intended work. (Vide PRACTICAL GEOMETRY. TO find the angles at the centre, and circumference of a polygon.) Extend these radii equally until the relative distances between them are of the length required to form the sides of the proposed equilateral redoubt.

5. Front of fortification, for a field fort.

Place pickets in a straight line, of the length required for the front of the proposed field work; from the centre of which drop a perpendicular inwards, making it for a square, pentagon, hexagon, or octagon, respectively one eighth, one fifth, one fourth, or one third of the exterior side. Direct the lines of defence from the termination of the exterior side to the end of the perpendicular, making the faces of the bastions two sevenths of the exterior side, and constructing the flanks perpendicular to, and joining the lines of defence. Other fronts are traced by laying down the exterior sides, at the angle of the circumference of the intended polygon (Vide PRACTICAL GEOMETRY) by means of the prismatic compass, &c., and then proceeding as directed for the former front.

REMARKS, AND GENERAL RULES.

1. The size of a work depends in general upon the number of men who are to defend it. If labour is the sole object of attention, the advantage must necessarily be the greater in proportion as the size of the work is less; but if the accommodation of the troops is

only to be considered, the advantage depends greatly upon occupying much ground.

2. The form of the work should be such as to contain the greatest surface with the least perimeter. By an adherence to this maxim, we obtain the greatest accommodation for the troops with the least labour. The form of a field work seldom depends upon choice, but generally upon the spot where it is to be raised, the purposes for which it is to be constructed, and the nature of the ground in the vicinity.

3. The interior of the work ought to be so covered by the parapet, that the men within, except when on the banquette, may not be seen from any part without, at the distance of cannon shot.

4. The circumjacent ground (to as great a distance as possible) ought to be cleared, that the enemy may not conceal, or shelter himself against the fire from behind the parapet. The nearer to the work that the enemy can find cover, the more advantageously he can form his dispositions; and, as his attacks may consequently be made with greater vigour, and be more readily supported, the success will be the more probable.

5. The flanking parts ought to be sufficiently capacious to contain all the men required for the defence of the flanked portions of the work.

6. The flanking parts ought to have nearly a direct view of those flanked; that is, the defence should be nearly at right angles, the most advantageous angle being 100 degrees.

7. The parts flanked ought to be within musket fire of their flanking parts.

8. The fire ought to be equally distributed, that every part of the work may be equally defended.

9. The work ought to be equally strong in all its parts, that it may everywhere equally resist the assaults of the enemy; and the parapet should be thick enough to withstand the shot fired against it.

10. The dimensions of the parapet should not only be sufficient to secure, and cover the troops within the work, but ought also to be of such a form as to afford a full view of the enemy in his approach; and at the same time discover, as little as possible, the men employed for its defence.

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PART X.

BRIDGES, AND PONTOONS.

BRIDGES.

To find the number of planks required to form a float, to support a given weight.

1st. Find the content of one plank (vide Practical Geometry, Part 12), and multiply it by the specific gravity of the wood; the product will be the weight of the timber.

2nd. Multiply the same solid content by the specific gravity of water; the product will be the weight of an equal bulk of

water.

Then take the difference of these two products, or weight, and it will be the weight one piece of timber will support without sinking. Hence by Proportion, the number required to support the given weight may be found.

To find the number of casks required to form a raft to support a given weight.

1st. Find the solid content of one cask in cubic inches (vide Practical Geometry), and multiply it by the specific gravity of water; the product will be the weight of a quantity of water of equal bulk with the cask.

2nd. From this product, or weight, subtract the weight of the cask, and the remainder will be the weight it will support without sinking. Then by Proportion, the number required for the formation of the raft may be found.

To find the number of boats, or pontoons, required to support a given weight.

The burthen a boat, or pontoon, will support without sinking beyond a given depth (the form of the boat, or pontoon being known) must first be found, thus

1st. Find the solid content of the part to be sunk, in cubic feet (vide Practical Geometry, Part 12), and multiply it by the specific gravity of water (vide Gravity, Part 12).

2nd. Subtract this product from the weight of the boat, or pontoon, and the remainder will be the burthen it will support without sinking beyond the required depth.

Then by Proportion, the number required to support the given weight may be computed.

Note-In the construction of bridges, should a rope require to be extended across a rapid river, the coil should be placed in the boat (instead of on shore) and be paid out as the boat advances.

PONTOONS.

Those called Blanshard's (from their inventor, Colonel Blanshard, Royal Engineers), are of two descriptions.

1.-LARGE PONTOONS.

Displacement of water, 97 cubic feet, equals 6088 lb., or 54 cwt.

The buoyant power of a raft of two pontoons, its own weight deducted, is 77 cwt., about one-half of which is a safe load. Each raft, or one carriage load, forms 2 bays, or 20 ft. 8 in. of bridge: its own weight will sink it about 7 or 8 inches. The crew of a raft consists of 6 rowers, and 1 steersman.

At open order the bridge will pass cavalry, field artillery, or infantry, with closed files.

train.

At close order the bridge will pass any part of a heavy

Dimensions, and weight of cylindrical pontoons, manufactured in the arsenal at Woolwich.