Page 182 - J. C. Turner "History and Science of Knots"

A History of Life Support Knots 173

be taken to avoid capsizing the knot in front of the descender; forming a Girth
Hitch (#56,[3]); some designs of the descender are made to avoid this [14, p.
163].
Many descenders can be also used to allow the controlled lowering of a
load [15], 129, p. 71][33]; the descender is anchored at the top and the rope
is threaded through it, with the load hanging below; the friction substantially
lessens the force needed to hold the load.

Fig. 45. Twisted Hitch Fig. 46. Figure Eight Descender
Such an arrangement of rope and descender is not usually suitable for use
belaying a climber. A belay system must freely pay out the rope to the climber
but be prepared to hold instantly if the climber falls. A dynamic belay reduces
the impact force on both anchorage and falling climber by allowing the rope
to pay out under controlled friction to bring it to a halt gradually. Before
World War II the friction was usually provided by wrapping the rope round
the arms and across the back of the belayer. This system brought problems
after the falling person was held, perhaps swinging freely at the end of the
rope: the belayer needed to disentangle himself from the system in order to
take appropriate action but without further endangering the victim. Using
friction round a device held in front of the belayer, instead of round his body,
minimised this problem. The simplest device is a Belay Plate (Fig. 47) or
even the smaller eye of a Figure Eight Descender. This constrains the rope
as it passes round a belaying karabiner. In normal use the rope passes freely,
guided by the gloved hands, but in the event of a fall the free rope, the end
not leading to the climber is pulled back hard. This tends to jam the rope
against the device and provides friction [10, p. 122] [14, p. 58](27, p. 84] [29,
p. 54]. Once the victim has been held, it is simple to make the rope fast with
an appropriate hitch and disengage the belayer. Several similar devices have
been developed, leading the rope round more or less complex curves.