An object of class "rate"
represents a diver's
rate of ascent or descent, or the total time taken to
ascend or descend. Such objects are useful in describing
dives, especially in the function dive
. The value returned by descent
represents a descent rate
or descent time. Exactly one of the arguments speed
and
time
should be given. If speed
is given, this specifies
a fixed rate of descent, in metres per minute. If time
is given,
this specifies a fixed total time of descent, in minutes.