2524. A long syllable sometimes stands in place of a short. A syllable
thus used is called _irrational_ (marked >) because it destroys the
normal #ratio# between arsis and thesis. The foot which contains such a
syllable is itself called irrational. The most common irrational foot is
the _irrational spondee_ (-> when it stands for a trochee; >- when it
replaces an iambus), which is found in iambic, trochaic, and logaoedic
rhythms.
Probably the irrational long suffered a slight correption (2517), so
that its duration was between that of the ordinary long and that of a
short syllable.