terial; the breaking up of
the hard soil of barren fields into soft loam ready to receive the
seed; the harvesting of the ripe grain, etc.

[Illustration: FIG. 91.--Prying a stone out of the ground.]

The more intelligent races among men soon learned to help themselves
in these tasks. For example, our ancestors in the field soon learned
to pry stones out of the ground (Fig. 91) rather than to undertake the
almost impossible task of lifting them out of the earth in which they
were embedded; to swing fallen trees away from a path by means of rope
attached to one end rather than to attempt to remove them
single-handed; to pitch hay rather than to lift it; to clear a field
with a rake rather than with the hands; to carry heavy loads in
wheelbarrows (Fig. 92) rather than on the shoulders; to roll barrels
up a plank (Fig. 93) and to raise weights by ropes. In every case,
whether in the lifting of stones, or the felling of trees, or the
transportation of heavy weights, or the digging of the ground, man
used his brain in the invention of mechanical devices which would
relieve muscular strain and lighten physical labor.

If all mankind had depended upon physical strength only, the world
to-day would be in the condition prevalent in parts of Africa, Asia,
and South America, where the natives loosen the soil with their hands
or with crude implements (Fig. 94), and transport huge weights on
their shoulders and heads.

[Illustration: FIG. 92.--The wheelbarrow lightens labor.]

Any mechanical device (Figs. 95 and 96), whereby man's work can be
more conveniently done, is called a machine; the machine itself never
does any work--it merely enables man to use his own efforts to better
advantage.

[Illustration: FIG. 93.--Rolling barrels up a plank.]

150. When do we Work? Whenever, as a result of effort or force, an
object is moved, work is done. If you lift a knapsack from the floor
to the table, you do work because you use force and move the knapsack
through a distance equal to the height of the table. If the knapsack
were twice as heavy, you would exert twice as much force to raise it
to the same height, and hence you would do double the work. If you
raised the knapsack twice the distance,--say to your shoulders
instead of to the level of the table,--you would do twice the work,
because while you would exert the same force you would continue it
through double the distance.

[Illustration: FIG. 94.--Crude method of farming.]