lustration: FIG. 103.--Primitive man tried to lighten his task by
balancing his burden.]

Prop the board so that the end _A_ is 2 feet above the table level;
that is, arrange the inclined plane in such a way that its length is
twice as great as its height. In that case the steady pull on the
balance will be one half the weight of the roller; or a force of 6
pounds will suffice to raise the 12-pound roller.

[Illustration: FIG. 104.--Less force is required to raise the roller
along the incline than to raise it to _A_ directly.]

The steeper the incline, the more force necessary to raise a weight;
whereas if the incline is small, the necessary lifting force is
greatly reduced. On an inclined plane whose length is ten times its
height, the lifting force is reduced to one tenth the weight of the
load. The advantage of an incline depends upon the relative length and
height, or is equal to the ratio of the length to the height.

156. Application. By the use of an inclined plank a strong man can
load the 600-pound bowlder on a wagon. Suppose the floor of the wagon
is 2 feet above the ground, then if a 6-foot plank is used, 200 pounds
of force will suffice to raise the bowlder; but the man will have to
push with this force against the bowlder while it moves over the
entire length of the plank.

Since work is equal to force multiplied by distance, the man has done
work represented by 200 x 6, or 1200. This is exactly the amount of
work which would have been necessary to raise the bowlder directly. A
man of even enormous strength could not lift such a weight (600 lb.)
even an inch directly, but a strong man can furnish the smaller force
(200) over a distance of 6 feet; hence, while the machine does not
lessen the total amount of work required of a man, it creates a new
distribution of work and makes possible, and even easy, results which
otherwise would be impossible by human agency.

157. Railroads and Highways. The problem of the incline is an
important one to engineers who have under their direction the
construction of our highways and the laying of our railroad tracks. It
requires tremendous force to pull a load up grade, and most of us are
familiar with the struggling horse and the puffing locomotive. For
this reason engineers, wherever possible, level down the steep places,
and reduce the strain as far as possible.

[Illustration: FIG. 105.--A well-graded railroad bed.]

The slope of the road is called its grade, a