e pulling down bodies to the earth
existed. An apple falls from a tree. Why, it does exist! There is
gravitation, common gravity that makes bodies fall and gives them their
weight.

Wanted, a force tending towards the centre of the earth. It is to hand!

It is common old gravity that had been known so long, that was perfectly
familiar to Galileo, and probably to Archimedes. Gravity that regulates
the motion of projectiles. Why should it only pull stones and apples?
Why should it not reach as high as the moon? Why should it not be the
gravitation of the sun that is the central force acting on all the
planets?

Surely the secret of the universe is discovered! But, wait a bit; is it
discovered? Is this force of gravity sufficient for the purpose? It must
vary inversely with the square of the distance from the centre of the
earth. How far is the moon away? Sixty earth's radii. Hence the force of
gravity at the moon's distance can only be 1/3600 of what it is on the
earth's surface. So, instead of pulling it 16 ft. per second, it should
pull it 16/3600 ft. per second, or 16 ft. a minute.[17] How can one
decide whether such a force is able to pull the moon the actual amount
required? To Newton this would seem only like a sum in arithmetic. Out
with a pencil and paper and reckon how much the moon falls toward the
earth in every second of its motion. Is it 16/3600? That is what it
ought to be: but is it? The size of the earth comes into the
calculation. Sixty miles make a degree, 360 degrees a circumference.
This gives as the earth's diameter 6,873 miles; work it out.

The answer is not 16 feet a minute, it is 13.9 feet.

Surely a mistake of calculation?

No, it is no mistake: there is something wrong in the theory, gravity is
too strong.

Instead of falling toward the earth 5-1/3 hundredths of an inch every
second, as it would under gravity, the moon only falls 4-2/3 hundredths
of an inch per second.

With such a discovery in his grasp at the age of twenty-three he is
disappointed--the figures do not agree, and he cannot make them agree.
Either gravity is not the force in action, or else something interferes
with it. Possibly, gravity does part of the work, and the vortices of
Descartes interfere with it.

He must abandon the fascinating idea for the time. In his own words, "he
laid aside at that time any further thought of the matter."

So far as is known, he never mentioned his disappointment to a soul. He
might