Source : Webster's Revised Unabridged Dictionary (1913)

Breastwheel \Breast"wheel`\ (br[e^]st" hw[=e]l`), n.
   A water wheel, on which the stream of water strikes neither
   so high as in the overshot wheel, nor so low as in the
   undershot, but generally at about half the height of the
   wheel, being kept in contact with it by the breasting. The
   water acts on the float boards partly by impulse, partly by
   its weight.