The circular-saw is not a perfectly flat disc, but when at rest is slightly convex on one side and concave on the other.

This fullness can be pushed back and forth as can the bottom of an oil-can.

When moving at a high rate of speed, however, the saw flattens itself by centrifugal force.

This enables it to cut straight with great accuracy.
[Illustration: Fig.32.Double Circular-Saw and Carriage.] A gang-saw is simply a series of straight saw-blades set in a vertical frame.

This has a reciprocating motion, enabling it to cut a log into a number of boards at one time.