But how do we get the cars back?  It is pleasant sliding down hill on a rail, but who pulls the sled back?  Gravitation.  It is just as willing to work both ways as one way.

Think of a great letter X a dozen miles long.

Lay it down on the side against three or four rough hills.  Bend the X till it will fit the curves and precipices of these hills.  That is the double track.  Now when loaded cars have come down one bar of the X by gravity, draw them up by a sharp incline to the upper end of the other bar, and away they go by gravity to the other end.  Draw them up one more incline, and they are ready to take a new load and buzz down to the bottom again.

I have been riding round the glorious mountain sides in a horseless, steamless, electricityless carriage, and been delighted to find hundreds of tons of coal shooting over my head at the crossings of the X, and both cars were drawn in opposite directions by the same force of gravity in the heart of the earth.

If you do not take off your hat and cheer for the superb force of gravitation, the wind is very apt to take it off for you.

THE FAIRY DRAWS GREATER LOADS

Pittsburg has 5,000,000 tons of coal every year that it wishes to send South, much of it as far as New Orleans—­2,050 miles.  What force is sufficient for moving such great mountains so far?  Any boy may find it.

Tie a stone to the end of a string, whirl it around the finger and feel it pull.  How much is the pull?  That depends on the weight of the stone, the length of the string, and the swiftness of the whirl.  In the case of David’s sling it pulled away hard enough to crash into the head of Goliath.  Suppose the stone to be as big as the earth (8,000 miles in diameter), the length of the string to be its distance from the sun (92,500,000 miles), and the swiftness of flight the speed of the earth in its orbit (1,000 miles a minute).  The pull represents the power of gravitation that holds the earth to the sun.

If we use steel wires instead of gravitation for this purpose, each strong enough to support half a score of people (1,500 pounds), how many would it take?  We would need to distribute them over the whole earth:  from pole to pole, from side to side, over all the land and sea.  Then they would need to be so near together that a mouse could not run around among them.

Here is a measureless power.  Can it be gotten to take Pittsburgh coal to New Orleans?  Certainly; it was made to serve man.  So the coal is put on great flatboats, 36 x 176 feet, a thousand tons to a boat, and gravitation takes the mighty burden down the long toboggan slide of the Ohio and Mississippi Rivers to the journey’s end.  How easy!

[Illustration:  The Head of the Toboggan Slide.]

One load sent down was 43,000 tons.  The flatboats were lashed together as one solid boat covering six and one half acres, more space than a whole block of houses in a city, with one little steamboat to steer.  There is always plenty of power; just belt on for anything you want done.  This is only one thing that gravitation does for man on these rivers.  And there are many rivers.  They serve the savage on his log and the scientist in his palace steamer with equal readiness.