[Larger plate.]

[531]. Twist drill for metal.

[532] & [533]. Countersinking drills for wood.

[534]. Diamond drill for rock; bores an annular hole, the core of which breaks out at intervals.

[535] to [545]. Well boring tools for different kinds of strata; tools for raising broken rods, &c.

[546]. Hollow boring cutter for cutting a shoulder on a central core; dowelling bit.

Section 31.—DIFFERENTIAL GEAR.

Devices to utilise the difference of velocity, or power, between two distinct moving parts.

[547]. Equational box. Two drivers A, A′, equally speeded in opposite directions, will drive the bevil gear at same velocity without revolving the spur wheel C′ which is loose on the shaft; but any alteration in the relative speeds of A and A′, causes the bevil pinion to travel round, carrying the spur wheel C′ at a speed equal to half the difference of the two velocities. This gear is used on traction engines to drive the swivelling wheels round curves, where the proportionate velocities of the wheels will vary with the radius of the curve. In this application of the gear B is the driving shaft, and A and A′ the swivelling wheels.