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, and the grade itself is simply the number of feet the hill rises per mile.  A road a mile long (5280 feet) has a grade of 132 if the crest of the hill is 132 feet above the level at which the road started.

[Illustration:  FIG. 106.—­A long, gradual ascent is better than a shorter, steeper one.]

In such an incline, the ratio of length to height is 5280 / 132, or 40; and hence in order to pull a train of cars to the summit, the engine would need to exert a continuous pull equal to one fortieth of the combined weight of the train.

If, on the other hand, the ascent had been gradual, so that the grade was 66 feet per mile, a pull from the engine of one eightieth of the combined weight would have sufficed to land the train of cars at the crest of the grade.

Because of these facts, engineers spend great sums in grading down railroad beds, making them as nearly level as possible.  In mountainous regions, the topography of the land prevents the elimination of all steep grades, but nevertheless the attempt is always made to follow the easiest grades.

158.  The Wedge.  If an inclined plane is pushed underneath or within an object, it serves as a wedge.  Usually a wedge consists of two inclined planes (Fig. 107).

[Illustration:  FIG. 107.—­By means of a wedge, the stump is split.]

A chisel and an ax are illustrations of wedges.  Perhaps the most universal form of a wedge is our common pin.  Can you explain how this is a wedge?

159.  The Screw.  Another valuable and indispensable form of the inclined plane is the screw.  This consists of a metal rod around which passes a ridge, and Figure 108 shows clearly that a screw is simply a rod around which (in effect) an inclined plane has been wrapped.