Editor’s Notes

This Report is based on a rough draft found amongst the
effects of the late Don Coase, one of the ‘leading lights’ of the B.E.C. in its
earlier years and who was responsible for the construction of most of the club
tackle at that time.  That he did not
live to complete this Report is yet another reason, amongst so many, for
regretting his untimely death.  However,
Norman Petty – with assistance from Alan Sandall – undertook the not easy task
of completing another person’s manuscript. Caving Report No. 10 is the result.

The date of the original draft is not known but it must have
been before the publication of Caving Report No.3, "The Manufacture of
Lightweight Caving Ladders", because this did not appear until four months
after Coase’s death.  It is possible that
he had seen the manuscript of Report No. 3 and this seems probable from some of
the points made.  In this case the draft
must have been written during 1957, this in any case being the most probable
date.

As already stated, only a very rough draft was available and
this, in places, has had to be altered slightly.  However, the aim throughout has been to keep
it in the style and intention that it is thought Don would have desired.  The draft covered Sections 1, 2, 3, 7 &
9, and part of Section 5, and luckily a list of the intended contents.  Norman Petty has written the remaining
Sections and some of the drawings were prepared by Alan Sandall.

The points where Caving Reports Nos. 3 and 10 overlap are
very few as they cover different methods of caving ladder construction.  In fact it is felt that this Report and the
revised edition of No. 3 are complementary end between them give a good insight
to the construction of caving tackle.

 

1.  INTRODUCTION

The need for a lighter and more compact ladder for caving
has always been obvious to anyone who had to carry a soaking wet wood and rope
ladder in and out of any but the easiest of caves.  Probably the earliest of lightweight ladders
were made by dÙJoly in

France

during the 1930’s where the need was more urgent than in this country.   Ladders often had to be transported up
several thousand feet of mountain side on rough tracks and the amount of ladder
repaired for some of the bigger French potholes would probably be enough to
ladder every pitch on Mendip.

The use of lightweight tackle did not cone into general use
in

Britain

until after the Second World War, probably due to the increased availability,
and technique of using, aluminium alloys developed in the aircraft
industry.  The Bristol Exploration Club
had previously produced some lightweight ladder which was not entirely
satisfactory, but with the opening of St. Cuthbert’s Swallet late in 1953 with
its large number of pitches the need was pressing for a considerable amount of
new tackle.  The decision was made to
concentrate on lightweight ladders, not only from the ease of transport but
also in view of the higher factor of safety, ease of inspection and increased
life when left underground for long periods. In actual fact during the earlier stages of exploring St. CuthbertÙs
ladders were left in the cave for up to three months without any signs of
corrosion or other deterioration.

This Report is intended to put on record the experience of
the B.E.C. in designing such tackle and it is felt that if it prevents anyone
else committing some of the mistakes the B.E.C. made, it will have served a
useful purpose.  Also included is a
description of a piece of equipment designed for a specific purpose in St. Cuthbert’s,
the Hand Climbing Line, or as it is more popularly known by club members, the
‘Knobbly Dog’.  This is a new idea as far
as the writer knows and may prove useful in other caves.  In conclusion are some remarks dealing with
the use of tackle.

 

2.  ORIGINAL B.E.C. LIGHTWEIGHT
LADDER

 

Figure 1

This was made in about 1942 or 1943 when the only transport
available from

Bristol

to Mendip was bicycles.  It consisted, of
aluminium alloy tubes 1/4" diameter for rungs and the wires were
1/16" diameter Bowden cable.  The
attachment of the rungs to the wire was simple if nothing else.  The rungs were drilled right through at
either and, the wire pulled out through the open end and tied in a clove hitch
round a 2 B.A. brass bolt.  The nut was
tightened up and the whole lot, knot, nut and bolt soft soldered
tug-ether.  In practice it worked
reasonably well with the small amount of use to which it was put but the wire
was too small and started rusting.  Also,
with the kinking of the wire around the bolt, the pull did not come in a
straight line and the wire started cutting through the thin wall of the
rungs.  The writer suggests that the
factor of safety was about zero!  Another
failing was that having been left rolled up for a considerable period, on the
only occasion that the writer used it, after having stopped off at the bottom
of the 40 Foot Pot in Swildons Hole, the lower section rolled itself up and
hung eight feet up the pitch.  However,
it was light and small enough to go in a bicycle saddle bag.  The method of rung fixing is shown in figure
1.

 

3.  B.E.C. STANDARD LIGHTWEIGHT
LADDER- Materials & Preparation

The first ladders built to this pattern were constructed in
1949-50.  There were a number of faults
in these early ladders but it is believed that these have been overcome.  The descriptions given are amplified by the
drawings of the component parts.

End Plugs.  These are-made from 1/2" diameter
Duralumin rod and are cut 1" long. They are drilled and tapped-along their length to take the 1/4"
Whitworth grub-screws that are used. Duralumin is preferable to aluminium as it is considerably easier to
drill and, in particular, to tap the holes.

One of the major defects of the earlier ladders was the
design of the end plugs.  They were made
as shown in figure 2(a) and due to the tapped hole stopping as shown the
grub-screw, when tightened, forced the wire into the aluminium.  Under load this deformed and let the rung
slip, down the wire.  One remedy that was
tried was to insert a plug of aluminium between the grub-screw and the wire as
it was thought that the tapping was not deep enough but this was not
successful, the rung still slipping.  The
ladder was eventually dismantled and the grub screw holes drilled and tapped
right through, as the present design, then reassembled using new wire.  After the initial bedding-in, no more trouble
was experienced.  With this method the
wire is forced into the hole and given a kink when the screw is tightened.   This kink is not severe enough to weaken the
wire appreciably but does positively prevent slipping of the rungs.  The present method is shown in figure 2(b).



Figure 2

Rungs.   These were originally constructed of
3/4"o.d. x 1/16" wall aluminium alloy tubing but a cheaper substitute
was found by using 3/8"o.d. commercial aluminium electrical conduit.  This suffered from the drawback that the wall
thickness is greater than 16 gauge and a press of some sort is needed to insert
the end plugs.  All later ladders were
constructed from 5/8"o.d. x 16 gauge wall alloy, tube and in this case,
the end plugs are made from 1/8" rod. All other dimensions of rungs are as stated on figure 3.         To facilitate the construction of the
large number of rungs required, a cutting and drilling jig has been constructed
for 5/8"o.d. rungs.  This is shown,
in figure 4 and is case hardened all over and is surface ground on both
sides.  The rungs are cut to length by
sliding in a length of tubing flush with one end and then cutting down close to
the other end with a hacksaw, finally filing the end square against the
jig.  The burr on the outside of the rung
can either be removed by filing or by rotating the rung against a sanding disc
of a 3/4" electric drill.  At the
same time the rung end should be chamfered. The burr inside the rung is best cleaned by rotating a round nosed
parallel rotary file exactly 1/2" diameter in a 1/4" electric drill
fitted in a bench stand, then feeding the rung end on to the rotary file for
not more than 1/8".  This provides a
starting guide when pressing the plugs in.



Figure 3

The next operation is to insert the end plugs.  The plugs for the earlier ladders were a
sliding fit in the tube and this was found to be a serious drawback as after
the wire holes were drilled the plugs slid out of line.  For this reason the practice has since been
adopted of using 16 gauge (0.064") wall thickness.  With 5/8” o.d. tube the inside diameter is
0.497" and this gives an interference fit of 0.003" when using
1/2" diameter plugs.  The plugs are
pressed into position, one at a time, by using a carpenterÙs cramp as shown in
figure 5.



Figure 4



Figure 5

After inserting the plugs the rung is placed in the cutting
and drilling jig and the holes for the wire drilled using a 1/8"
drill.  If the rungs are made of Dural or
a similar hard alloy the holes are slightly countersunk to remove the sharp
edges.  It has been found that some of
the individual strands of wire in contact with the hole broke after a few
months use if this was not done.

Grub Screws.  These are 1/4" Whitworth x 1/4"
long hexagon socket grub screws, preferably of "Allan" or
"Unbrake" manufacture.  The
type of end of these screws varies, figure 6, but the only one readily
available is the half or reversed cup point (a).  Although this is not ideal it seems to serve
well in practice.  There is one variation
of this that must not be used and this is where the cup point is serrated.  This, when tightened, cuts into the
individual wires of the rope.  The
preferred type is the cone point of 60° (b), if this can be obtained, as this
causes no damage to the wire.



Figure 6

Wire Rope.  Originally this was 10cwt aircraft cable of
7/19 construction but now 15cwt cable of similar construction is used as the
cost is the same.  This type of cable is
extremely flexible and is made of seven strands, arranged as (a) in figure 7,
each strand being made from nine teen wires (b), and each individual wire,
approximately 0.01" diameter is galvanised.  On no account should a wire rope be used with
a fibre core as this acts as a sponge holding water in the centre of the rope
and thus accelerates corrosion. (Editor’s Note: Fibre cored wire ropes have been used satisfactorily for
caving ladders without corrosion – see Caving Report No. 3A.)



Figure 7

End Fittings
(C-Links)
.  Considerable variations
exist in the type of end fittings but the most popular is the C-Link and this
has been standardised for all B.E.C. ladders and ancillary equipment.  The simplest is undoubtedly a link cut from a
piece of chain.  Various chains were cut
and tested on a tensile testing machine and the low figure at which most links
failed was rather surprising.   Loads in
the order of four or five hundredweight resulted in the links opening right out
as shown in figure 8.  Tests were
continued until a section of 3/8" close pitch chain was found to stand 520
pounds before any opening of the gap occurred. The loading was slowly increased with the gap gradually widening until
at a load of just over nine hundredweight the link opened fully with no
increase of load.  The link showed no
sign of fracturing.  This chain, which
was manufactured from E.N. 8 steel, was considered to be more than adequate for
the purpose and a length was obtained – sufficient for several hundred
links.  As the facilities were available
these links were cadmium plated and this does in fact give a finished look to
the completed ladders.



Figure 8

Thimbles.  The wire end is passed round a tinned iron or
stainless steel thimble which contains the C-Link.  The readily available thimbles are designed
for 1/2" circumference rope which is slightly larger than 10 or 15cwt.
aircraft cable.  The correct thimble for
the aircraft cable seems to the writer to be rather small and flimsy and so the
1/2" circumference thimble has been used, being flattened to the diameter
of the rope used.

Wire End Fixing.  Two methods have been used for finishing off
the ends of the wire.  The earlier
ladders using 10cwt cable were clamped and soldered in a ferrule.  All the later ladders using 15cwt cable have
been spliced.



Figure 9

With the ferrule method a one inch long piece of
1/4"o.d. copper pipe is carefully cleaned and flattened in a vice to the
shape shown in figure 9, just sufficiently to slide two sections of the wire
through it.  The ferrule is then
thoroughly tinned inside and out.  This is
important as otherwise corrosion could be rapidly set up between the copper and
zinc coating of the wire.  The wire has
then to be tinned either side of the thimble for an inch only, and the free end
cut so that it stops just at the end of the ferrule.  To assemble, slide the ferrule on to the
wire, then pass the wire round the thimble containing a C-Link and finally slip
the end of the wire into the ferrule again. After pulling the wire tight round the thimble, the ferrule is squeezed
as flat as possible in a vice and then the wire-ferrule assembly is soldered
up.  When soldering care must be taken
not to apply any more heat than necessary and under no account must a naked
flame be used as otherwise the temper of the wire is lost.  For the same reason a tin-lead solder is also
preferred.  It will be obvious that a
non-corrosive flux is essential and for this purpose "Alkaray" flux,
which is approved by the Aeronautical Inspection Department as being
non-corrosive, has been used on all the later ladders.  It is also necessary to clean the wire well
before soldering and methylated spirits, carbon tetrachloride or a similar
solvent are required to remove the oil incorporated in the rope during
manufacture.  The "Alkaray"
flux has none of the penetration of, say, ”killed spirits" and in
practice has been found difficult to solder rope which has been kept in stock
for a period when the zinc coating has tarnished.  (Editor’s note: This method of forming end
loops is not to be recommended as the long term effects of the solder on the
rope are not known, see the revised edition of Caving Report No. 3.)

Eye splicing the cable end seems to be the safest method as
it is possible to inspect the splice periodically by removing the binding,
whereas with the ferrule method it is possible for the wire to corrode in the
ferrule and the first warning is when the ladder breaks in service.  The difficulty of splicing the cable is not
great and anyone can splice a hemp rope can, after a little practice, make a
fair splice in wire rope.  However, it is
a tedious job and painful on the fingers. The writer, who is by no means an expert at the job, finds it takes
approximately an hour to make one splice. The writer’s technique is first to bind the cable five inches from the
free end and then open out the strands back to the binding and solder the cut
ends of each strand for 1/8" to prevent the individual wires coming
apart.  Baker’s Fluid or
"Fluxite" can be used for this as the ends will be cut off
later.  Then assemble the cable round the
thimble containing the C-Link.  The cable
should be bound to the thimble, or the method the writer finds easier, using
the oversize thimbles, is to close over slightly the outside of the thimble so
that-the wire is held securely.  Then
remove the binding from the free end of the cable.  Various ways are possible for making the
first round of tucks and the method adopted by the author is that given by the
British Standard Institute for splicing wire ropes.   Details are given in the Appendix.

The British Standards Institute specify a five round splice
three rounds of full strands and the last two with approximately half of the
wires cut out, thus causing the finished splice to taper.  The writer, however, prefers to make four
full rounds and then two with halved strands just to be on the safe side.  These additional rounds of tucks are made in
exactly the same way as the second round, being pulled tight and beaten after
each round.  Finally the surplus wire is
cut off sand the whole splice served (or bound) with either waxed twine or
small diameter (20 gauge maximum) galvanised wire.  The purpose of the binding is to hold the
whole splice tight and to cover the short projecting cut ends of wire.  The waxed twine is preferable as it leaves
the splice relatively flexible but it is liable to chafe or rot through under
cave conditions unless frequently inspected and renewed.

One point that is worth taking some trouble over is getting
the distance of the ends, from the rungs, correct.  This should be 5 1/2" from the centre of
the end rung to the inside of the far end of the C-Link so that when the
ladders are joined the rung spacing remains at an 11" pitch.  If care is not taken it will be found that
one side will hang lower than the other, which makes it disconcerting when
climbing the finished ladder.

Another method of finishing the ends is the
"Talurit" splicing process. This is similar to the ferrule method except that the ferrule is a thick
aluminium sleeve which is swaged on to the rope .by means of a hydraulic
press.  This process is usually done
commercially and tends to be expensive, costing 2/11d per splice although a
reduction in price is made when more than six are done at one time.  In comparison, the charge for a hand splice,
made commercially, is 1/9d irrespective of the number.

 

4.  B.E.C. STANDARD LIGHTWEIGHT
LADDER – Assembly

Having prepared all the component parts of the ladder as
described in Section 3 the wire rope sides are cut to length allowing several
inches spare.  One end is soldered solid
for approximately two inches and then filed down so that it slides easily
through the holes drilled in the rungs. As this end of the rope is cut off after threading, "Baker’s
Fluid" may be used as the flux for soldering.

C-Links are spliced on the unsoldered ends of the two wire
ropes and the first rung threaded on and positioned 5 1/2 inches from the
centre of the rung to the inside of the far end of the C-link.  The grub screws are then tightened to clamp
the rung.  The remaining rungs are
clamped three or four at a time using either the jig shown in figure 10 or else
a wooden spacer 10 3/8" long.  When
the last rung has been fixed the soldered ends are cut off and the C-links
spliced in position.



Figure 10

At this stage the wires and splices are treated with a good
rust preventative such as "Tokall". This is obtainable from Croda Ltd, Cowick Hall, Snaith, GOOLE, Yorks at
17/6 per gallon, including carriage and can be applied by brush, oil-can or
dipping.  The author finds an oil can
very effective, dipping being messy and requiring a large volume of
liquid.  The ladder is left to dry for
two or three days before the splices are served.

All the grub screws are checked to ensure, that they are
really tight and then to protect them against rust and mud the screw sockets,
exposed threads and ends of the rungs are filled with hot candle wax.  The surplus wax is removed by lightly
countersinking with a 5/16" drill. When in use the ladders should be treated with "Tekall" or a
similar solution not less than twice a year – this being applied when the
ladders are clean and dry.

Rung and Wire Spacing.   The rung spacing has been standardised at
11"; this being about the practical maximum.  Climbing a ladder with a 12" pitch is
quite an effort, whilst some ladders of another club with rungs at 15" or
16" pitch proved impossible to a novice who had to be pulled up and a very
strenuous gymnastic feat for an experienced caver.

The spacing between the wires has been fixed at
6".  This gives plenty of room for
the normal climbing boot complete with nails.

Ladder Lengths.  The ladders are made in lengths of twenty and
forty feet, these being considered convenient lengths.  It is felt that standardised, lengths should
be made to avoid confusion, it being annoying to reach a pitch and find that
the ladder is too short because two ladders used were each five feet shorter
than it was thought.

 

5. WOOD AND WIRE LADDERS

When lightweight metal ladders were first put into service a
few club members complained that when climbing these after a strenuous wet
cave, the aluminium rungs caused cramp in their hands.  This was no doubt due to the high thermal
conductivity of the metal, so several lengths of ladder were made using wire
rope and wooden rungs.  An experimental
ten feet length was first built and when this proved satisfactory, a further fifty
feet were made.  These were not replaced
when they wore worn out as the original complaints had ceased, probably due to
the fact that in St. Cuthbert’s Swallet, the scene of much of the club’s
activities for the last few years, the pitches had been fitted with rigid metal
ladders.

The general design can be seen from figure 11.  The main consideration was to make them as
simple as possible using only simple hand tools.  The rungs are made of ash, 7" x 1"
x 7/8" having all the corners removed, and drilled at six inch centres to
take 10cwt wire rope.  They are supported
on aluminium sections made by cutting 5/8" x 5/8" diameter plugs in
half to form semi-cylindrical pieces, and drilled to make a sliding fit on the
cable.  A short length of tinned 18swg wire
was passed through the centre of the wire rope, bent parallel to the rope and
soft soldered to take the weight.  A
similar solder globule above the rung enabled the rung to be slid up the wire
for inspection of the rope but at the same time preventing the rung from
getting too far out of position. Obviously this type of ladder could only be hung from one end.



Figure 11

 

6. SPECIAL PURPOSE ULTRA-LIGHTWEIGHT LADDER

(Editor’s note: This section had not been written by Coase,
and Petty does not know the method of construction.  Very clear diagrams had been drawn, however,
and these are reproduced as figures 12 – 14. As the reproduced diagrams are not as clear as the originals the
following interpretation of them is given.)

The ladder was constructed using 10cwt, 7/14 construction
cable and had rungs of 6" overall length with a rung pitch of 11",
the distance between the wire centres being 5 1/4".  The rungs were oval in cross-section,
3/8" x approximately 11/16" (the latter figure is not given on the drawing
and has been obtained by measurement of the rung as drawn) but whether this
cross-section was purchased or formed from circular tubing is not known.  Similarly the wall thickness of the tubing is
not shown but appears to be approximately 14 gauge.  (Don Coase was a draughtsman and it is
assumed that his original diagram was drawn at least approximately to scale.)

Each rung was prepared by drilling, through one side only,
two holes at 5 1/4" centres with a No. 32 drill.  On the opposite side of the rung was cut an
1/8" wide slot, 7/32" deep and at an angle of 45° to the length of
the rung.  This is shown in figure 12.

The method of rung fixing appears to have been as
follows.  Rungs and 1/4" diameter
copper ferrules were threaded alternately on to the wire rope, then at 11"
centres the ferrules were squashed flat and soldered.  A rung was centred over each ferrule by
threading the flattened ferrule through the slot in the bottom of the rung and
finally the rung was clamped by flattening the end tightly over the
ferrule.  This method of fixing would
necessitate the ladder being hung



Figure 12

from the correct end so that the rung was forcing the
ferrule against the drilled hole and not against the slot.  The fixing of the rungs is shown in figure
13.



Figure 13

The third diagram, figure 14, shows the jig used for
slotting and drilling the rungs.  It was
manufactured from 3/4" x 1/8" angle iron and made to fix on a bench
vice in place of the hardened jaws.  The
inside measurements of the angle iron were cut to 13/16" x 11/32"
from the 3/4" x 3/4" and they are so arranged in the vice that they
form, in cross-section, a hollow rectangle into which the rung fits.  Appropriate guide slots and drilling hole were
than made.

 

Figure 14

Note:  A ladder of
very similar dimensions is used by the Shepton Mallet Caving Club for normal
use but these use the ‘taper pin’ method of rung fixing. ‘The method of
manufacture has been fully described in Caving Report No. 3A.

 

7.   TETHERS AND SPREADERS.

Tethers.  (See figure 15.)  These are simply a length of 15 cwt aircraft
cable with the ends finished off by any of the methods suggested for ladders,
each end being fitted with a C-link. Standard lengths are five, ten and twenty feet long.  Although it has not been done to date, it is strongly
recommended that a loose C-link should be incorporated in each tether, free to
slide up and down the wire, to enable the tether to be fixed round a rock or
other suitable belay and leaving the end free to carry a spreader and then a
ladder.  (See remarks under ‘Use of
Tackle’, below.)



Figure 15



Figure 16

Spreaders.  (See figure 16.)   These are similar in construction to tethers
except that they are made with a fixed C-link in the centre.  Even if splicing of the ends is adopted, this
centre link requires the wire to be fixed by a tinned copper sleeve or, as an
alternative, seized for at least one inch with galvanised soft iron wire, or
tinned copper wire, and then run over with solder.

 

8.   THE USE OF TACKLE.

Due to the short ends of the ladder, if the sides are joined
together to a tether, a considerable bending stress is imposed on the wire
where it enters the first rung – see figure 17(a).  This is extremely detrimental and leads, to
early failure of the wire at the bend point. Another bad practice is to shackle the two sides of the ladder together
and fix them over a "Rawlbolt" or stalagmite boss, figure 17(b).  This was done by one club member on the Forty
Foot in Swildons and led to one wire breaking whilst the ladder was being climbed.  (Examination of the broken portions of the
wire gave increased faith in the soldered ferrule type end fixing as the
ferrule had been bent into almost a semi-circle round the "Rawlbolt"
without any sign of the wire pulling out.)

 

Figure 17

For these reasons, it is essential to use either a spreader
or to connect either side independently to the ends of a tether, figures 17(c)
and (d).  When using a tether and
shackling back to the loose C-link it is essential to avoid bending the wire
more than absolutely necessary; see figure 18. A further point when shackling two lengths of ladder together is to make
sure that there are no twists in the wire before joining the C-links.  It should be obvious that neither the ladder
wires nor the tethers must be bent over sharp edges of rock.  Any permanent kink in the wire will reduce
the life very considerably and the rock will probably rub through the zinc coating
of the wire when the wire is repeatedly stressed with people climbing the
ladder.



Figure 18

 

APPENDIX I

The Splicing of Six Stranded Ropes.

Reference:       British Standards Institution Handbook No. 4, Part 1, pages 178
– 182 (1958 edition).

Place the thimble in a vice with the rope vertical.  The main part of the rope should be on the
right hand and the tail strands on the left. Seize the thimble at the crown and on both flanks.

The length of the tails for a five tuck splice should be
four inches for each 1/8" diameter of the rope.

First series of tucks.  (See figures 19 and 20.)

The tail strands are numbered 1-6 anti-clockwise and the
spaces between the strands of the main part of the rope are lettered A – F,
also anti-clockwise.

A fibre main core should be tucked with tail No. 1 and then
cut off.  A wire mean core must be split
up, distributed amongst the tails and tucked with then for at least three
series.



Figure 19

 

 

Tail No.

In at:

Out at:

 

 

 

1

B

A

 

 

 

6

C

B

 

 

 

2

B

C

 

 

 

3

C

D

 

 

 

4

D

F

 

 

 

5

D

E

 

 

 

 

 

 

 

 

 

Second Series

 

 

Third Series

 

Tail No.

In at:

Out at:

Tail No.

In at:

Out at:

1

B

C

1

D

E

6

C

D

6

E

F

2

D

E

2

F

A

3

E

F

3

A

B

4

F

A

4

B

C

5

A

B

5

C

D

 

After the third series of tucks, the wires of a wire main
core may be broken off and the number of wires in each of the main tails
reduced to half the original number, preferably by ‘breaking out’.  To ‘break out’ wires take each wire
separately, snatch back to the point where it emerges from the rope and then
twist the wire – handle fashion – reversing direction if necessary and the wire
should part in the gusset.  The remaining
wires should be twisted up to a rough strand formation and at the same time this
should enclose the cut ends in the centre. After reducing the number of wires in the strands the fourth and fifth
series of tucks are made.

 

Fourth Series

 

 

Fifth Series

 

Tail No.

In at:

Out at:

Tail No.

In at:

Out at:

1

F

A

1

B

C

6

A

B

6

C

D

2

B

C

2

D

E

3

C

D

3

E

F

4

D

E

4

F

A

5

E

F

5

A

B

 



Figure 20

 

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