Page:Popular Science Monthly Volume 46.djvu/627

Rh which is called a "cycloidal line," and may be described as that line produced by a point revolving about a moving center. The shape of the curved line depends upon the relative rate of speed of the two motions, the circular one of the point and the forward

one of the plate. If the latter is comparatively slow, the cycloid will take this shape (Fig. 8); if still slower, the curves will cross each other instead of nearly touching. If the motion of the plate is comparatively rapid, the cycloid will take the form of Fig. 9, or one still more open.

By repeating these lines and causing them to overlap each other beautiful lacelike effects can be had, very difficult to imitate by hand (Fig. 10). The patterns can be varied almost endlessly.

Machine engraving of a far more intricate character is produced by the "geometrical lathe," which is one of the most delicate and complex machines ever invented (Fig. 11).

Americans have particularly distinguished themselves in mechanical engraving, and, in fact, it was one of our people—a Mr. Spencer, of Philadelphia who introduced the "bank-note engraving machine" in the early part of this century. This machine, however, was very primitive, as it had but one "cam," and consequently was very limited in its possibilities as compared

with those built in the present day, which have seven cams as well as other attachments not dreamed of by Spencer.

The geometrical lathe of the present day can not be described so as to be perfectly understood, but something of an idea of its working can be gained from the following:

A tool, mechanically sharpened and shaped, made of hardened steel, is used on this machine and is fixed solidly and immovably in a rest, or carrier, over the chuck of the lathe. If the chuck