[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.]

Lifting heavy weights through great distances is not the only way in which work is done.  Painting, chopping wood, hammering, plowing, washing, scrubbing, sewing, are all forms of work.  In painting, the moving brush spreads paint over a surface; in chopping wood, the descending ax cleaves the wood asunder; in scrubbing, the wet mop rubbed over the floor carries dirt away; in every conceivable form of work, force and motion occur.

A man does work when he walks, a woman does work when she rocks in a chair—­although here the work is less than in walking.  On a windy day the work done in walking is greater than normal.  The wind resists our progress, and we must exert more force in order to cover the same distance.  Walking through a plowed or rough field is much more tiring than to walk on a smooth road, because, while the distance covered may be the same, the effort put forth is greater, and hence more work is done.  Always the greater the resistance encountered, the greater the force required, and hence the greater the work done.

The work done by a boy who raises a 5-pound knapsack to his shoulder would be 5x4, or 20, providing his shoulders were 4 feet from the ground.

The amount of work done depends upon the force used and the distance covered (sometimes called displacement), and hence we can say that

       Work = force multiplied by distance,
  or W = f x d.

151.  Machines.  A glance into our machine shops, our factories, and even our homes shows how widespread is the use of complex machinery.  But all machines, however complicated in appearance, are in reality but modifications and combinations of one or more of four simple machines devised long ago by our remote ancestors.  These simple devices are known to-day, as (1) the lever, represented by a crowbar, a pitchfork; (2) the inclined plane, represented by the plank upon which barrels are rolled into a wagon; (3) the pulley, represented by almost any contrivance for the raising of furniture to upper stories; (4) the wheel and axle, represented by cogwheels and coffee grinders.