Undershot Wheels. Those which are acted upon by a stream at the bottom; overshot ones acting by a stream at the top. Undershot wheels require a much greater body of water than overshot; the former are consequently constructed in rivers, and the latter in streams of water. The application of water to mechanical purposes in the grinding of corn, fulling of cloth, dressing of leather, spinning of cotton, wool, etc, &C, deserves to be ranked among the most important inventions of man.