The Collaborative International Dictionary of English v.0.48:

Incommensurable \In`com*men"su*ra*ble\, a. [Pref. in- not +
   commensurable: cf. F. incommensurable.]
   Not commensurable; having no common measure or standard of
   comparison; as, quantities are incommensurable when no third
   quantity can be found that is an aliquot part of both; the
   side and diagonal of a square are incommensurable with each
   other; the diameter and circumference of a circle are
   incommensurable.
   [1913 Webster]

         They are quantities incommensurable.     --Burke.
   -- In`com*men"su*ra*ble*ness, n. -- In`com*men"su*ra*bly,
   adv.
   [1913 Webster]