Focus of, 236; rectilinear, _245_; spherical aberration in, 243. Levers, signal, colours of, 230. Speed governors, 67, _68_, _69_; Hartwell, 70; marine, 71. Speed of motor car, 103, _104_. Graham, 418. Gramophone, 317; records, 319, 321; reproducer, _318_. Hairspring, 412. Hay-cutter, 451. Heart, the, 355; disease, 361; rate of propagation does not know till the other.