A principle put in practice and applied to some art, machine, manufacture, or composition of matter. Earle v. Sawyer, 4 Mason, 1, Fed. Cas. No. 4,247. See Act Cong. July 8, 1870. In the law of patents, this term means a useful art or manufacture which is beneficial and which is described with exactness in its mode of operation. Such an art can be protected only in the mode and to the extent thus described. Smith v. Downing, 22 Fed. Cas. 511; Carnegie Steel Co. v. Cambria Iron Co.