Chain harrows.

Brush harrows.

Plank or drag harrows.

These types vary in the depth to which they cut, and the degree to
which they pulverize the soil.

_Rolling cutter harrows._ Harrows of this type (see Fig. 54) consist
of one or more revolving shafts on which are arranged a number of
concave disks. These disks are either entire, notched, or made of
several pieces fastened together. Examples of these are the disk,
cutaway and spading harrows. These harrows cut and move the soil
deeper than the other types. They are especially adapted to work on
heavy clay soils.

The value of this type of harrow as moisture preservers depends on the
manner in which they are used. If the disks are so set that they cover
but a portion of the surface with a mulch of fine earth they leave a
ridge exposed to the action of the wind and sun and the rate of
evaporation is greatly increased. The disks should be set at such an
angle that the whole surface shall be stirred or covered. Soils which
need the disk harrow should generally be gone over again with some
shallower working tool to smooth the surface. An objection to the
rolling cutters is that unless great care is taken they will leave the
land in ridges and valleys.

The two gangs of disks throw the earth in opposite directions. They