The Seconds Pendulum

An interesting historical fact from these MIT notes: Christiaan Huygens proposed defining the meter conveniently as the length of a pendulum that produces a period of 2 seconds. A pendulum’s period is

\displaystyle  T = 2\pi \sqrt{\frac{l}{g}},

so, using Huygens’ standard of T = 2s for 1 meter,

\displaystyle  g = \frac{4\pi ^{2}\times 1\ \textup{meter}}{4s^{2}} = \pi ^{2}ms^{-2}.

“So, if Huygens’s standard were used today, then g would be π2 by definition.”

An Elevated Perspective

tetrahedron example

Consider a triangle ABC and three other triangles (ABD1, BCD2, and ACD3) that share common sides with it, and assume that the sides adjacent to any vertex of ABC are equal, as shown. The altitudes of the three outer triangles, passing through D1, D2, and D3 and orthogonal to the sides of ABC, meet in a point.

This can be made intuitive by imagining the figure in three dimensions. Fold each of the outer triangles “up,” out of the page. Their outer vertices will meet at the apex of a tetrahedron. Now if we imagine looking straight down at that apex and folding the sides down again, each of those vertices will follow the line of an altitude (from our perspective) on the way back to its original position, because each follows an arc that’s orthogonal to the horizontal plane and to one of the sides of ABC. The result is the original figure.

(Alexander Shen, “Three-Dimensional Solutions for Two-Dimensional Problems,” Mathematical Intelligencer 19:3 [June 1997], 44-47.)


The symptoms of a typical attack
A clearly ordered sequence seldom lack;
The first complaint is epigastric pain
Then vomiting will follow in its train,
After a while the first sharp pain recedes
And in its place right iliac pain succeeds,
With local tenderness which thus supplies
The evidence of where the trouble lies.
Then only — and to this I pray be wise —
Then only will the temperature rise,
And as a rule the fever is but slight,
Hundred and one or some such moderate height.
‘Tis only then you get leucocytosis
Which if you like will clinch the diagnosis,
Though in my own experience I confess
I find this necessary less and less.

From Zachary Cope, The Diagnosis of the Acute Abdomen in Rhyme, 1947.

More Loops

Further to my March post “A Lucrative Loop,” reader Snehal Shekatkar of S.P. Pune University notes a similar discovery of iterates leading to strange cycles among natural numbers.

Here is a simple example. Take a natural number and factorize it (12 = 2 * 2 * 3), then add all the prime factors (2 + 2 + 3 = 7). If the answer is prime, add 1 and then factorize again (7 + 1 = 8 = 2 * 2 * 2) and repeat (2 + 2 + 2 = 6). Eventually ALL the natural numbers greater than 4 eventually get trapped in cycle (5 -> 6 -> 5). Instead of adding 1 after hitting a prime, if you add some other natural number A, then depending upon A, numbers may get trapped in a different cycle. For example, for A = 19, they eventually get trapped in cycle (5 -> 24 -> 9 -> 6 -> 5).

For some values of A, several cycles exist. For example, when A = 3, some numbers get trapped in cycle (5 -> 8 -> 6 -> 5) while others get trapped in the cycle (7 -> 10 -> 7).

(Made with Tian An Wong of Michigan University.) (Thanks, Snehal.)

Podcast Episode 341: An Overlooked Bacteriologist,_Calcutta,_1894_Wellcome_L0037329.jpg
Image: Wikimedia Commons

In the 1890s, Waldemar Haffkine worked valiantly to develop vaccines against both cholera and bubonic plague. Then an unjust accusation derailed his career. In this week’s episode of the Futility Closet podcast we’ll describe Haffkine’s momentous work in India, which has been largely overlooked by history.

We’ll also consider some museum cats and puzzle over an endlessly energetic vehicle.

See full show notes …

Cutting Cake

mabry proof

In the College Mathematics Journal in 2001, Rick Mabry published this “proof without words” that

\displaystyle  \frac{1}{3} + \frac{1}{3^{2}} + \frac{1}{3^{3}} + \cdots = \frac{1}{2}.

He gives a charming explanation here.

(Rick Mabry, “Mathematics Without Words,” College Mathematics Journal 32:1 [January 2001], 19.)

First Things First

It is a traditional axiom of medicine that health is the absence of disease. What is a disease? Anything that is inconsistent with health. If the axiom has any content, a better answer can be given. The most fundamental problem in the philosophy of medicine is, I think, to break the circle with a substantive analysis of either health or disease.

— Christopher Boorse, “Health as a Theoretical Concept,” Philosophy of Science 44:4 (1977), 542-573

Podcast Episode 340: A Vanished Physicist

In 1938, Italian physicist Ettore Majorana vanished after taking a sudden sea journey. At first it was feared that he’d ended his life, but the perplexing circumstances left the truth uncertain. In this week’s episode of the Futility Closet podcast we’ll review the facts of Majorana’s disappearance, its meaning for physics, and a surprising modern postscript.

We’ll also dither over pronunciation and puzzle over why it will take three days to catch a murderer.

See full show notes …

Road Games

Statistics textbooks sometimes ask: Suppose you’re driving on the highway and adjust your speed so that the number of cars you pass is equal to the number that pass you. Is your speed the median or the mean speed of the cars on the highway?

The expected answer is that it’s the median speed, since the number of cars traveling more slowly than you is equal to the number traveling faster. But California State University mathematician Larry Clevenson and his colleagues wrote in 2001, “This certainly is true of the cars that you see, but that isn’t what the problem asks, and it isn’t the correct answer.”

Surprisingly, they found that the correct answer is the mean. “If you adjust your speed so that as many cars pass you as you pass, then your speed is the mean speed of all the other cars on the highway.” Details at the link below.

(Larry Clevenson et al., “The Average Speed on the Highway,” College Mathematics Journal 32:3 [2001], 169-171.)

Choosing Sides

shekatkar image

Temple University anthropologist Wayne Zachary was studying a local karate club in the early 1970s when a disagreement arose between the club’s instructor and an administrator, dividing the club’s 34 members into two factions. Thanks to his study of communication flow among the members, Zachary was able to predict correctly, with one exception, which side each member would take in the dispute.

The episode has become a popular example in discussions of community structure in networks, so much so that scientists now award a trophy to the first person to use it at a conference. The original example is known as Zachary’s Karate Club; the trophy winners are the Zachary’s Karate Club Club.

(Wayne W. Zachary, “An Information Flow Model for Conflict and Fission in Small Groups,” Journal of Anthropological Research 33:4 [1977], 452-473.) (Thanks to Snehal Shekatkar for the image.)