## “Boat Moved by a Rope”

There is a form of boat-racing, occasionally used at regattas, which affords a somewhat curious illustration of certain mechanical principles. The only thing supplied to the crew is a coil of rope, and they have, without leaving the boat, to propel it from one point to another as rapidly as possible. The motion is given by tying one end of the rope to the after thwart, and giving the other end a series of violent jerks in a direction parallel to the keel. I am told that in still water a pace of two or three miles an hour can be thus attained.

The chief cause for this result seems to be that the friction between the boat and the water retards all relative motion, but it is not great enough to affect materially motion caused by a sufficiently big impulse. Hence the usual movements of the crew in the boat do not sensibly move the centre of gravity of themselves and the boat, but this does not apply to an impulsive movement, and if the crew in making a jerk move their centre of gravity towards the bow

ntimes more rapidly than it returns after the jerk, then the boat is impelled forwards at leastntimes more than backwards: hence on the whole the motion is forwards.

— W.W. Rouse Ball, *Mathematical Recreations and Essays*, 1905

## Math Notes

2 × 5 × 27 = 1 × 15 × 18

2 + 5 + 27 = 1 + 15 + 18

213 × 624 = 312 × 426

102 × 402 = 201 × 204

936 × 213 = 639 × 312

## Viviani’s Theorem

Pick any point inside an equilateral triangle and measure the distances to its sides.

The sum of those distances is the altitude of the triangle.

## Seeing and Believing

John Dalton was a tornado of English science, exploring atomic theory, meteorology, perception, and the physics of gases with equal avidity.

But he was a Quaker, and when in 1834 he was invited to be presented to William IV, the question arose whether he could properly appear in the scarlet robes of an Oxford doctor of laws, as the color was forbidden to him.

Dalton solved this neatly: He pointed out that he was color-blind. “You call it scarlet,” he said. “To me its color is that of nature — the color of green leaves.”

## Free Falling

Published in 1869, Edward Everett Hale’s story “The Brick Moon” described the launch of an artificial satellite nearly a century before Sputnik:

If from the surface of the earth, by a gigantic peashooter, you could shoot a pea upward from Greenwich, aimed northward as well as upward; if you drove it so fast and far that when its power of ascent was exhausted, and it began to fall, it should clear the earth, and pass outside the North Pole; if you had given it sufficient power to get it half round the earth without touching, that pea would clear the earth forever. It would continue to rotate above the North Pole, above the Feejee Island place, above the South Pole and Greenwich, forever, with the impulse with which it had first cleared our atmosphere and attraction. If only we could see that pea as it revolved in that convenient orbit, then we could measure the longitude from that, as soon as we knew how high the orbit was, as well as if it were the ring of Saturn.

Because the 200-foot brick sphere is accidentally launched with human occupants, Hale perhaps also deserves credit for anticipating the space station.

## Paper Work

Rutgers mathematician E.P. Starke posed this question in the *American Mathematical Monthly* of July 1940:

“In high school geometry texts and elsewhere one frequently meets the statement that the reason for the straightness of the crease in a folded piece of paper is that the intersection of two planes is a straight line. This is fallacious. What is the correct reason?”

I was going to post this as a puzzle, but after much pondering I’ve been unable to make sense of the answer. Here it is:

“Let *P*, *P*′ be two points of the paper that are brought into coincidence by the process of folding. Then any point *A* of the crease is equidistant from *P*, *P*′, since the lines *AP*, *AP*′ are pressed into coincidence. Hence the crease, being the locus of such points *A*, is the perpendicular bisector of *PP*′.”

I agree that this is true, but I don’t see what’s wrong with the first answer. Any ideas?

UPDATE: The consensus seems to be that the first answer makes some invalid assumptions, including flat planes and Euclidean space, where Starke’s proof is more rigorous. Thanks to everyone who’s written in.

(Second update, on reflection: Presumably the books that Starke mentions were not claiming that all creases *must* be straight, only that a straight crease is so because two planes intersect in a line. That still seems reasonable to me.)

## Charmed

A visitor to Niels Bohr’s cottage noticed a horseshoe nailed over the door.

“Surely you don’t expect that a horseshoe will bring good luck?” asked the visitor.

“No, I don’t,” Bohr said. “But they say it works even if you don’t believe in it.”

See The Misfortune Field.

## The Look and Say Sequence

What’s the key to this curious sequence of numbers?

1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, …

When read aloud, each term describes the one that precedes it. The first term consists of “one 1,” the second of “two 1s,” the third of “one 2, then one 1,” and so on.

That seems pretty arbitrary, but it opens a door into an unsuspected mathematical universe. Start with any number (except 22, an obvious dead end) and it will produce a string of digits that lengthens by about 30 percent with each generation — indeed, the percentage approaches a predictable constant (30.3577269 …) as the length approaches infinity.

More amazingly, the growing string will organize itself into a series of recognizable finite substrings that evolve predictably with each generation. John Horton Conway, who discovered all this, identified 92 such substrings, which he named after the chemical elements. Thus “uranium” (3) decays into “protactinium” (13), which becomes “thorium” (1113), and so on.

Thus an infinitely complex universe can arise from simply reading the number 1 aloud.

## The Euler Characteristic

For any convex polyhedron,

vertices – edges + faces = 2

In 1988, readers of the *Mathematical Intelligencer* judged this the world’s second most beautiful theorem — behind Euler’s identity.

## Misc

- Georgia, Massachusetts, and Connecticut didn’t ratify the Bill of Rights until 1939.
- Wilt Chamberlain never fouled out of a game.
- 3864 = 3 × (-8 + 6
^{4}) - What’s the opposite of “not in”?
- Alaska has a longer coastline than all other U.S. states combined.
- “To do nothing is also a good remedy.” — Hippocrates

## A Better Nature

Trisect the angles of any triangle and you’ll find an equilateral triangle at its heart.

This theorem has a curious cousin: If you trisect the sides of any triangle and erect an equilateral triangle outwardly on the middle third of each leg, then the outermost vertices of these equilateral triangles will themselves form an equilateral triangle.

## The Thatcher Effect

When we look at another person’s face, her eyes and mouth convey the most information about her mood.

Indeed, when a face is inverted we can have trouble recognizing it because we can’t read its expression.

So in 1980 University of York psychologist Peter Thompson tried inverting everything *but* the eyes and mouth.

Most people can recognize the face at left and assign a mood to it, but they’re often surprised to see it right side up.

“Further research into this illusion might help determine whether face recognition is a serial or parallel process,” Thompson wrote in *Perception* that summer. “It might even tell us something about Margaret Thatcher.”

## Scholars and Sense

O Cleinias, are those who learn the wise or the ignorant?

He answered that those who learned were the wise.

Euthydemus proceeded: There are some whom you would call teachers, are there not?

The boy assented.

And they are the teachers of those who learn — the grammar-master and the lyre master used to teach you and other boys; and you were the learners?

Yes.

And when you were learners you did not as yet know the things which you were learning?

No, he said.

And were you wise then?

No, indeed, he said.

But if you were not wise you were unlearned?

Certainly.

You then, learning what you did not know, were unlearned when you were learning?

The youth nodded assent.

Then the unlearned learn, and not the wise, Cleinias, as you imagine.

— Plato, *Euthydemus*

## Never Mind

“It has been asserted (by C.S. Lewis, for instance) that no determinist rationally can believe in determinism, for if determinism is true, his beliefs were caused, including his belief in determinism. The idea seems to be that the causes of belief, perhaps chemical happenings in the brain, might be unconnected with any reasons for thinking determinism true. They might be, but they need not be. The causes might ‘go through’ reasons and be effective only to the extent that they are good reasons.”

— Robert Nozick, “Reflections on Newcomb’s Paradox,” 1974

“If … [determinism] is true, then the intellectual or cognitive operations of its upholders, including their choice or decision to maintain the thesis, … are themselves only the effects of inexorable forces. But if this is so, why should the thesis … be accepted as valid or true?”

— Alan Gewirth, *Reason and Morality*, 1978

## Pandigital Approximations

The digits 1-9 can work some impressive tricks:

The first formula, found by B. Ziv in 2004, produces the first 10 digits of pi.

The second, astonishingly, reproduces *e* to 18,457,734,525,360,901,453,873,570 decimal places. It was discovered by Richard Sabey, also in 2004.

(Thanks, Robin.)

## Party of One

“A man wrote to say that he accepted nothing but Solipsism, and added that he had often wondered it was not a more common philosophy. Now Solipsism simply means that a man believes in his own existence, but not in anybody or anything else. And it never struck this simple sophist, that if his philosophy was true, there obviously were no other philosophers to profess it.”

— G.K. Chesterton, *St. Thomas Aquinas*, 1933

## “Triangle Rhyme”

Although the altitudes are three,

Remarks my daughter Rachel,

One point’ll lie on all of them:

The orthocenter H’ll.

By mathematician Dwight Paine of Messiah College, 1983.

(Further recalcitrant rhymes: *month*, *orange*. W.S. Gilbert weighs in.)

## Misc

- River Phoenix was born River Bottom.
- Every natural number is the sum of four squares.
- What happens if Pinocchio says, “My nose will grow now”?
- Shakespeare has no living descendants.
- “All generalizations are dangerous — even this one.” — Dumas

## Math Notes

1000000000000000000000000000000000000000000000000000000000000000

0000000000000000000000000000000000000000000000000000000000000000

0000000000000000000000000000000000000000000000000000000000000000

000002569 is prime.

## You Are Here

In “Partial Magic in the *Quixote*,” Borges writes:

Let us imagine that a portion of the soil of England has been levelled off perfectly and that on it a cartographer traces a map of England. The job is perfect; there is no detail of the soil of England, no matter how minute, that is not registered on the map; everything has there its correspondence. This map, in such a case, should contain a map of the map, which should contain a map of the map of the map, and so on to infinity.

This sequence tends to a single point, the point on the map that corresponds directly to the point it represents in the territory.

Cover England entirely with a 1:1 map of itself, then crumple the map into a ball. So long as it remains in England, the balled map will always contain at least one point that lies directly above the corresponding point in England.

See Garganta and Papered Over.

## Half-Hearted

Draw a semicircle and surmount it with two smaller semicircles.

A line drawn through A, at any angle, will divide the perimeter precisely in half.

This probably has some romantic symbolism, but I’m not very good at that stuff.