 
CORDAGE.Ropes are distinguished as to size by their circumference in inches. Their length is given in fathoms. Hemp rope when new stretches freely, and deteriorates very much after a few months' wear. It is about one third stronger than manilla rope. To ascertain the strain in pounds a rope will bear without breaking, square the circumference and multiply the result by the tabular unit in the following table:
For ropes in daily use reduce the tabular unit one third to meet reduction in strength by wear and exposure. A safe general rule for all ropes is, one fourth the square of the circumference gives the breaking weight in short tons. TABLE OF WEIGHTS THAT MANILLA ROPE IN DAILY USE WILL SUSTAIN.
To Preserve White Rope.—Dip when dry into a bath containing 20 grms. (about seven tenths of an ounce) of sulphate of copper per litre (a little over one quart) of water; soak for four days and dry. Then soak in a solution of 100 grms. (3.5 oz.) of soap per litre of water. Iron-wire Rope.—Its breaking weight in tons is about equal to the square of the circumference in inches. Steel-wire Rope.—It is from 2 to 2½ times stronger than iron-wire rope. Wire rope can be safely worked in field service to one half its breaking weight. To Preserve Wire Rope.—Apply linseed-oil with a piece of sheepskin, wool inside; or mix the oil with equal parts of Spanish brown and lampblack. If used underground or in water, take mineral or vegetable tar, add one bushel of fresh-slaked lime to barrel of tar, boil it well, and then saturate the rope with the boiling tar. Chains.—Chains, when used in place of ropes, should be examined to see that every link is free from flaws and not too much worn. The chain should have no kinks in it, and each return, when more than one is used, should bear its proper strain. Do not work a chain over a windlass of SAFE WORKING STRAIN FOR FIELD SERVICE, AND WEIGHT IN FATHOMS.
BLOCKS, TACKLE, ETC.Blocks are of two kinds, made and mortised. A made block consists of four parts: shell, sheave, strap, and bush. A mortised block is made of a single block of wood, mortised out to receive a sheave. Blocks are single, double, or treble, according to the number of sheaves. A Tackle is a purchase formed by reeving a rope through two or more blocks for the purpose of hoisting. A Whip is a purchase made by a rope rove through one single block. A Gun-tackle Purchase is a rope rove through two single blocks, and made fast to the strap of the upper block. The parts of all tackles between the fasts and sheave are called the standing parts. The parts between the sheaves are the running parts, and the part which is taken hold of in hoisting is called the fall. A Whip upon Whip is where the block of one whip is made fast to the fall of another. A Luff-tackle Purchase is a single and a double block, the end of the rope being fast to the upper part of the single block, and the fall coming from the double block. A luff-tackle upon the fall of another luff-tackle is called luff upon luff. A Watch-tackle or Tail-tackle is a luff-tackle purchase, with a hook in the end of the single block, and a tail to the upper end of the double block. A Single Burton is composed of two single blocks, with a hook in the bight of the running part. Reeve the end of rope through the upper block, and make it fast to the strap of the fly-block. Then make fast your hook to the bight of the rope, and reeve the other end through the fly-block for a fall. The hook is made fast by passing the bight of the rope through the eye of the hook and over the whole. This is a very quick-working tackle and a strong purchase. Used for hoisting entirely. When a very heavy weight is to be raised, the standing part should be attached to the slings by a fisherman's bend instead of to the block. The size of blocks is expressed by the length of the shell in inches. Tackles are also designated by the number of sheaves employed, as twofold (two single blocks), threefold (double and single block), etc. The bight of a hook is the middle of the bend of the hook part. Rope should always be stopped up, either with the end or with rope-yarn stops, to prevent its getting into a snarl. When using ropes for hauling, they should never be dragged upon the ground. Before reeving a rope in a block the turn should be carefully taken out, to prevent twisting when the weight is lifted. This is done by stretching the rope out to its full length, and turning it in the opposite direction to that in which it is laid up until all the stiffness disappears. Blocks should be overhauled very often to see that the sheaves are working properly on the pins, and that they work smoothly. If they do not, turn the pins end for end and rub a little black lead (graphite) on them to lubricate them; also on the sides of the sheaves where they rub against the shell. When hoisting with tackles, they should never be allowed to twist. If they show a tendency to do so, insert a bar in the block or sling, and use it as a lever to hold it straight. It frequently happens that the men cannot apply their full strength in the direction in which it would be most effective. In such cases hook a single block to some object about two feet above the ground and reeve the end of the fall through it, so that the men can add their strength to their weight, and more men can apply themselves. Never trust the suspension of a weight to holding it by the unaided strength of men. If it is possible to get a turn around any fixed object, even in raising or hauling a weight, it is best to take a turn, as all that is gained is then saved. Always select such blocks that the fall will run freely through them, and not ride upon the edges of the sheaves. If it does, it will be certain to cut. The rope should not quite fill the groove on the sheave. In this way excessive friction is avoided. The power gained by using tackle is as follows: Two single blocks, or gun-tackle: nearly doubled. Luff-tackle, double and single block: doubled. If the double block is movable, trebled. Two double blocks: power × 3?. Double and treble blocks: power × 4. Two treble blocks: power × 4½. Whip upon whip (single Burton): trebled. When one tackle is applied to another, the power obtained is found by multiplying their respective values together. No advantage is gained by using a greater number of sheaves than two treble blocks in one fall. The power is equal to the weight divided by the number of ropes (standing parts) attached to the lower block, or by twice the number of rising pulleys (sheaves). KNOTS, HITCHES, ETC.Thumb-knot (Fig. 108).—Used to prevent the end from unfraying, or to prevent its slipping through a fall. Figure-eight Knot (Fig. 109).—Used to prevent the end from unfraying, or to prevent its slipping through a fall. Fig. 108. Fig. 109. Fig. 110. Fig. 111. Fig. 112. Reef-knot (Fig. 110).—For joining two ends of rope or chain together. Singlesheet Bend (Fig. 111).—For joining dry ropes of unequal size. Doublesheet Bend (Fig. 112).—Same for wet ropes. Draw-knot (Fig. 113).—Same when it may be required to cast them loose. Running-knot (Fig. 114).—To form a loop that will draw taut. Fig. 113. Fig. 114. Bowline (Fig. 115).—To form a loop at the end of a rope which will not slip. Half-Hitch.—For securing the loose ends of lashings. Two Half-hitches (Fig. 116).—For making fast a rope-end to an object. Clove-hitch (Fig. 117).—Two half-hitches, used for commencement and finish of lashings; making fast the end or the bight of a rope to any object. Fig. 115. Fig. 116. Fig. 117. Fig. 118. Timber-hitch (Fig. 118).—Used on pieces of timber, or in making fast to spars, where the weight will keep the rope taut. Round Turn and Two Half-hitches (Fig. 119).—For making fast a rope so that the strain shall not jam the hitches. Fisherman's Bend (Fig. 120).—For making fast where there is give and take motion, as when a boat is at anchor. Fig. 119. Fig. 120. Fig. 121. Lever-hitch (Fig. 121).—For drawing pickets, fixing rounds of rope-ladder, bars to, or loops on, drag-ropes. Man's Harness-Hitch (also Fig. 121).—Loop to pass over shoulder. To fix a rope with a weight on it rapidly to a block. Fig. 122. Fig. 123. Cat's-paw (Fig. 122).—At the end or in the middle of a rope for hooking on a block. Blackwall Hitch (Fig. 123).—Simple hitch with pliable rope and fixed weight. Used to transfer the strain from one rope to another, or to secure a tackle to a rope for a horizontal pull. Stopper Hitch (Fig. 124).—To shift the strain off a rope temporarily. Magnus Hitch (Fig. 125).—For making fast to round spars. Sheepshank (Fig. 126).—To shorten a rope without cutting it. Fig. 124. Fig. 125. Fig. 126. Fig: 127. LASHINGS.Square Lashing (Fig. 127).—Make fast one end (the fixed if there be one) of the lashing-rope to one of the objects to be lashed by a timber-or clove-hitch, or by passing the eye-end of the rope under it, bringing it up and passing the other end through the eye; then, after hauling taut, pass it round the second object, round the first, up again round the second, and down round the first, continuing this until a sufficient number of returns are made, when the whole of the returns are tightened by bringing the end round the returns between the objects two or three times, and making fast with two half-hitches on either of the first returns or one of the objects. If the returns on one of the spars lie in succession outside the returns first applied, they must lie in succession inside those first applied on the other spar. Diagonal Lashing (Fig. 128).—Begin with a timber-hitch or running-bowline round both spars and draw them together; then take three or four turns across each fork and finish with frapping-turns. Wedges with well-rounded points are often useful for tightening lashings. Fig. 128. Fig. 129. Sheer Lashing.—The lashing is made fast to the lower spar, above where they cross, with a timber-hitch; as many turns are taken around both spars towards the feet as may be necessary to cover the cross, and the end is then Gin Lashing (Fig. 129).—The three legs of the gin are placed so that the centre one is pointing in the opposite direction to that of the others, all being parallel, the upper ends resting on a skid and two inches apart. The lashing is commenced with a clove-hitch on an outside spar and carried over and under loosely and without riding six times. Two frapping-turns are taken in each interval and the whole finished off with two half-hitches round one of the spars. Iron chain is much better than rope for these fastenings. Fishing Spars.—This consists in strengthening spars by lashing others parallel to them. The fishing spars should be placed against the spar to be fished so that they may take off as much strain and be in as close contact as possible. Lashing-ropes should then be applied round all of the spars until they are all firmly bound together. Frapping.—The drawing together the several returns of a lashing so as to tauten it. The rope used for frapping is generally the running-end of the lashing itself; after having frapped, the end of the rope is made fast by two half-hitches, one around all the returns, the other on half only. BRIDGES.Weight.—Infantry in column of fours when crowded is allowed per lineal foot 560 pounds. Cavalry under same conditions is allowed 700 pounds. Field-artillery in column of sections weighs 400 pounds per lineal foot, so that a bridge built for infantry will carry it. In calculating the strength of parts of bridges the Formula for weight that may be borne safely by a beam supported at both ends and loaded in the centre:
W being the weight in pounds, l the distance between the points of support, d the depth, and b the breadth, all expressed in inches. S is a coefficient whose value for different kinds of wood is given in the following table:
Strength of Round Timber.—The strength of a circular pole is six tenths that of a square beam whose side is equal to the mean diameter of the pole; so that the equation becomes
in which d is the mean diameter. The bridge must be capable of sustaining the weight when crowded; and while the load for infantry is usually distributed, the greatest strain is brought on the balks, or road-bearers, when the gun-wheels are at the centre of the bay; and a balk will only bear half the load concentrated at its centre that it will bear when distributed. Roadway and Approaches.-The roadway may be of planks 1½ to 2 inches for ordinary traffic, or of poles, fascines, hurdles, etc. Litter or earth should be scattered over it to deaden the rattle of planks, which is apt to frighten the horses, and preserve the roadway. If heavy loads are to be hauled, planks should be laid longitudinally to form wheel-tracks. Eight feet in the clear will suffice for width of roadway, but nine feet is to be preferred. A hand-rail should be provided, especially if horses are to pass, and ribbands on either side. At least five balks (road-bearers) A rise in the centre of the bridge, called the camber, is allowed for the subsequent settling. This allowance is usually one thirtieth of the bay. The approaches are very important; the exit must be fully as good as the entrance, to avoid crowding. If not, care must be taken not to admit troops at the entrance faster than they can pass off. Ramps at either end steeper than one on ten are inconvenient. TRESTLES, PIERS, ETC.Transoms.—They must be sufficiently strong to bear the weight that can be concentrated on one bay, reckoned as distributed load on the transom. On a 9-foot roadway for light artillery, with bays of 15 feet, the transoms should not be less than 9 inches in diameter. Trestles are used for bridges across shallow rivers having sound hard beds and not liable to sudden floods; and for gaps, etc. They are not suitable for deep muddy rivers. They may be used in streams 9 feet deep with velocity of 4 miles an hour; or in deeper streams if the current be less. Two-legged Trestle (Fig. 130).—The standards, transom, and ledger are marked at the proper intervals, the heights of the first having been obtained by a careful inspection, and the lengths of the two latter depending on the width of the proposed roadway. The transoms should be about 3 feet longer than the width of the roadway in the clear, and the standards of the trestle should have a splay outward of six on one. Square lashings must be used. The braces are put on the frame with both butts and one tip on the same side, the second tip on the reverse side; the butts can be lashed simultaneously with the ledger and transom. The frame must then be squared by testing the diagonals, Fig. 130. Placing Trestles.—They are most easily placed by hand; but with those with two legs only care must be taken that they do not fall over. Each frame should be fastened to either bank; cables or struts may be used. Three-legged Trestles (Fig. 131), two of which are required for a single transom, have the advantage of standing without bracing, admit of ready adjustment, and utilize light material. They are, however, unsuitable for an uneven bottom, are extremely difficult to place, excepting by hand, and the legs require weighting in water. To make them, lash two legs together by a sheer lashing, open them out and then add the third leg; the trestle must then be up-ended, the feet placed on the angles of an equilateral tri Fig. 131. Cribs, Gabions, and Fascines.—These are also used for supporting roadways, the two latter are particularly useful for short gaps. Limbers.—Bridges supported on transoms lashed across the tops of limber-wheels may be used. FRAME BRIDGES.If a trestle bridge is impracticable, owing to depth of gap or stream, unevenness of bottom, or swiftness of current, a frame bridge may be used. Fig. 132. Single Lock.—This is suitable for spans not exceeding 30 feet, measured between the footing of the frames. 1st. Prepare footings 18 inches wider apart on one side than the other. They must be correctly squared or the frames will not lock. 2d. Measure the gap and lay out section of it on the ground with a line and pickets, allowing for camber. 3d. Lay out the standards in the section, and chalk-mark them at the proper positions for lashing ledgers and transoms. 4th. Lash the frames in position on either side of the gap; butts of standards towards the bank, ledger lashed on the top of standards, about two feet from the butts, but position varied according to nature of footings, transom lashed underneath. The splay of legs in a frame is not so great as in a two-legged trestle, one foot difference between transom and ledger generally sufficing; the transom of narrow frame 18 inches wider than the width of roadway in the clear between ribbands. Square the frames carefully before lashing the braces; see that the distance apart of the butts corresponds to the footings, broad frame to be 18 inches wider than the narrow. Drive pickets or large piece of timber (bollard) for guys and foot-ropes. 5th. Attach fore and back guys and foot-ropes to each frame, the latter fastened with a timber-hitch round the standards below the ledgers. 6th. Pass across the fore guys, passing those of the narrow frame between the horns of the broad frame. 7th. Launch and lock the frames. 8th. Send out temporarily two road-bearers and by their means and with the aid of two men working on the crutch of the bridge get the fork-transom in position. 9th. Send out remainder of road-bearers, resting the whole on the fork-transom. 10th. Place planks (chesses); rack down; put up hand-rail. ESTIMATE OF TIMBER.
If the stores are at hand and ready for use, 1 non-commissioned officer and 16 men on each side should complete the bridge in a little more than an hour. Double-lock Bridge.—This is used for spans of 30 to 45 feet. In laying out the section and making the spars for the double-lock bridge it must be remembered that the road-bearing transoms, TT, in this case are not the transoms lashed to the frames, but those which are subsequently lashed on the top of the distance-pieces. Fig. 133. The frames are of equal widths. In making the bridge the frames after being launched are held by the back guys a little higher than their ultimate position. The distance ESTIMATE OF TIMBER.
Fig. 134. Single-sling Bridges, used to 50 feet.—The frames for a single-sling bridge have an upper (locking) and lower (road-bearing) transom. In laying out the section the frames must be made to lock at such a height as to have at Fig. 135. A Treble-sling Bridge has three slung transoms, one being slung from the standards on either side between the fork and road-bearing transoms. The process of construction is similar to that of a single-sling. If the frames are very long and heavy, it is well to brace them above as well as below the road-bearing transoms to prevent their getting racked out of shape in launching. The upper braces, if they interfere with the headway, can be removed ESTIMATE OF TIMBER.
For a treble-sling bridge standards from 50 to 60 feet long will be required; also additional spars for transoms, road-bearers, and vertical frames. THE STRINGER BRIDGE.If the span does not exceed 25 feet, abutments good, and proper timber near at hand, this bridge can be easily built. TABLE OF TOOLS AND MATERIALS FOR SINGLE-AND DOUBLE-LOCK AND UNSTIFFENED SINGLE-SLING BRIDGES.
Six stringers and fifty flooring-poles, 6 inches in diameter and 12 feet long, will be required. Two axemen to each stringer will fell and prepare them in a few minutes, and while they are being carried and placed in position the axemen can prepare the poles. Stringers should have 4 feet extra length. Their diameter will depend on the weight to be borne, how distributed, and kind of wood. It is determined by the formulas already given. A good rough rule for calculating the live load which can be borne by rectangular timber of given length and scantling is: For larch safe distributed live load in cwts. (112 lbs.)
in which b = breadth in inches, d = depth in inches, L = length of span in feet. For fir the load may be 4/3, for cedar 5/3, for beech, oak, and pitch-pine 2, and for teak 7/3 of load of larch. This formula gives a theoretical factor of safety of about 3. The stringers are placed in position by means of jumping-poles, each of which should be strong enough to bear the weight of the stringer and a little longer than the hypothenuse of the right-angled triangle, the base and perpendicular of which are respectively three fourths the width and depth of the chasm. Place first stringer on abutment as a temporary wall-plate and chock it; slide the second and third over the chasm a little more than one fourth their length for a footway; push out one jumping-pole, butt first, until nearly balanced on wall-plate stringer, and pass the bight of a rope around it a little in advance of its centre of gravity, by means of which a man on the footway supports the butt while the pole is being slipped forward until it reaches its proper resting-place (while this is being done the ends of the stringers forming the footway are held down); then place second jumping-pole in position; draw back the two stringers forming footway; cross jumping-poles, lashing them about 2 feet above the level of the abutment, and attach guy-rope long enough to reach across the stream; place stringer in crutch and push forward until nearly balanced; then raise end and push so as to throw it forward and cause the other end to rest on the opposite abutment. Four men now cross over on the stringer, steadying themselves by means of the guy-rope held taut for the purpose, lift the stringer off the jumping-poles, and, with assistance from opposite side, roll the stringer in position. Pull back jumping-poles and place the other stringers. The flooring is laid as for corduroy roads, and the bridge is finished by pinning on a ribband of poles to hold down the ends of the flooring and erecting hand-rails if required. FORDS, FLYING AND FLOATING BRIDGES.When reconnoitring a river with a view of effecting a passage, observe: the nature of the banks, the nature of the bed, position and depth of fords, strength of current, whether tidal or otherwise, probability and extent of floods. Fords.—The following depths are fordable: For infantry, 3 ft.; for cavalry, 4 ft.; wagons containing ammunition, 2? ft. Gravelly bottoms are best; sandy bottoms are bad, as the sand stirs up and increases depth of water. Fords should be clearly marked by long pickets driven into the bed of the river above and below the ford, their heads being connected by a strong rope. It is well to mark the pickets, in order that any rise of the water may be at once evident. The depth of a river is generally most uniform in straight parts; at bends the depth will generally be greater at the concave bank, and less at the convex. For this reason a river which is not anywhere fordable straight across may be found passable in a slanting direction between two bends. To measure the velocity of a stream, use a light rod weighted at the end so as to stand vertically in the water; note the distance it floats in a given number of seconds; then seven tenths the mean number of feet a second gives the number of miles an hour. Ferries and Flying Bridges.—The simplest form of permanent ferry consists of ropes stretched across the river by means of which rafts can be hauled from bank to bank. The flying bridge can be used if the velocity of the current is two miles an hour or more. The current moves the boat or raft across the stream by acting obliquely against its side, which should be kept at an angle of about Floating Bridges.—These can be made of boats, barrels, timber, etc. Each pier must have enough available buoyancy to support the heaviest load that can be brought on one bay of the bridge. The length of the piers should be at least twice the breadth of the roadway, for steadiness, and they should be connected together at their extremities by tie-balks or lashings. The waterway between the piers should never be less, and should if possible be more, than the width of those piers. In barrel-pier bridges, or with boats having strong gunnels and frames, each balk bears on both gunnels of adjacent piers; in weak boats they bear on a central beam supported on the keel. The roadway of floating bridges is similar to that already described. Boats.—Open boats should not be immersed deeper than within one foot of the gunnel, and a still larger limit of safety will be required in rough water or a violent current. Buoyancy.—This may be determined by loading the boat with unarmed men to such a depth as is considered safe. For bridging purposes the number of men multiplied by 160 gives the available buoyancy in pounds. If the number of men be divided by four, the result will be the central interval in feet at which the boats may be placed in the bridge to carry infantry in fours crowded. RAFTS OF CASKS OR BARRELS.Buoyancy of Casks.—The actual buoyancy is given by the formula 5c2l-W, in which c is the circumference of the cask in feet half-way between the bung and the extreme end; l is the length in feet, exclusive of projections, measured along a stave; and W is the weight of the cask in pounds. The available buoyancy for bridging purposes may be taken at 9/10 of the actual buoyancy. A safe buoyancy may be obtained by multiplying the content in gallons by 8?. Knowing the buoyancy of one cask, the number to be united in a raft to sustain any desired load can be readily calculated. Constructing the Float.—Having determined on the number of casks to be united in one float, take one third the number and place them transversely on two poles laid on the ground parallel to each other and one half the height of a cask apart; the casks to be distributed evenly over a distance equal to the width of the proposed bridge, Floats of casks, when in bridge, should always be rigidly connected with each other at their ends by stringers, which must be lashed to both gunnels of each float; the roadway stringers can then be laid and should rest squarely on both gunnels of each float and should be lashed, especially if there be any sway or for animal traffic. TABLE OF WEIGHTS, DIMENSIONS, CAPACITY, ETC.
American wine-barrels are about the same size and weight as whiskey-barrels. Rafts.—Raft bridges may be used in a current not exceeding four miles an hour. Cubic content of a log in feet is equal to the square of one fifth of the mean girth multiplied by twice its length. Weight of timber (dry) per cubic foot: Ash 47 lbs., beech 43, elm 36, fir 32, oak 54, pine 40, poplar 24, syca Flotation.—To obtain the flotation of a log, multiply its cubic content by the difference between its weight per cubic foot and the weight of a cubic foot of water (62½ lbs.). Take five sixths of the result for the available buoyancy. To Form a Raft.—Place the logs side by side, the small ends alternating; strongly secure with rope and, if possible, by cross and diagonal pieces of scantling fastened by spikes or treenails. If it is to be used as a pier, the logs may be placed in two layers to avoid obstructing the waterway. A central transom must be used. The up-stream end of the raft may, with advantage, be convex. Make-shift Anchors.—Two or more pickaxes lashed together; heavy weights, such as large stones or railway irons; the latter are best when bent. Nets filled with stones are particularly good on rocky bottoms. Protection.—Arrangements must be made up-stream to protect a floating bridge from damage from floating substances, either by a boat patrol, or by stretching a net, or some intercepting obstacle, across stream. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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