The Collaborative International Dictionary of English v.0.48:

Mesolabe \Mes"o*labe\, n. [L. mesolabium, Gr. ?; me`sos middle +
   ? to take.]
   An instrument of the ancients for finding two mean
   proportionals between two given lines, required in solving
   the problem of the duplication of the cube. --Brande & C.
   [1913 Webster]