The rotation of the earth thus fixes the directions of north and south and defines those of east and west.
Such rotation can obviously be controlled within limits that need not be further considered.
The opportunities which rotation cropping affords for the cleaning of the land from weeds is another distinct element of advantage.
Under hay are included the produce of clover, sainfoin and rotation grasses, and also that of permanent meadow.
The cups are placed symmetrically on the end of the arms, and it is easy to see that the wind always has the hollow of one cup presented to it; the back of the cup on the opposite end of the cross also faces the wind, but the pressure on it is naturally less, and hence a continual rotation is produced; each cup in turn as it comes round providing the necessary force.