Irtish, an enormous river of Western Siberia and chief tributary of the Obi; its course from the Altai Mountains runs NW. through the Siberian plains for 1200 m.; it is navigable almost all the way in summer, and in winter it is a highway for sledge traffic; on its banks stand Semipalatinsk, Omsk, and Tobolsk.
Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)
Irreducible Case * Irving, Edward