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.