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.