If three circles “kiss,” like the black circles above, then a fourth circle can be drawn that’s tangent to all three. In 1643 René Descartes showed that if the curvature or “bend” of a circle is defined as k = 1/r, then the radius of the fourth circle can be found by

The ± sign reflects the fact that two solutions are generally possible — the plus sign corresponds to the smaller red circle, the minus sign to the larger (circumscribing) one.

Frederick Soddy summed this up in a poem in Nature (June 20, 1936):

The Kiss Precise

For pairs of lips to kiss maybe
Involves no trigonometry.
‘Tis not so when four circles kiss
Each one the other three.
To bring this off the four must be
As three in one or one in three.
If one in three, beyond a doubt
Each gets three kisses from without.
If three in one, then is that one
Thrice kissed internally.

Four circles to the kissing come.
The smaller are the benter.
The bend is just the inverse of
The distance from the center.
Though their intrigue left Euclid dumb
There’s now no need for rule of thumb.
Since zero bend’s a dead straight line
And concave bends have minus sign,
The sum of the squares of all four bends
Is half the square of their sum.

To spy out spherical affairs
An oscular surveyor
Might find the task laborious,
The sphere is much the gayer,
And now besides the pair of pairs
A fifth sphere in the kissing shares.
Yet, signs and zero as before,
For each to kiss the other four
The square of the sum of all five bends
Is thrice the sum of their squares.

(Thanks, Sean.)

In January 1797 London tea broker James Tilly Matthews was committed to the Bethlem psychiatric hospital after increasingly erratic outbursts in which he claimed he was being persecuted by political enemies. In 1809 Matthews’ friends petitioned for his release, arguing that he was no longer insane, and Bethlem apothecary John Haslam published a book-length study showing how bad his case had become.

Matthews believed that a gang of spies were occupying a Roman wall near the asylum and torturing him with a device called an air loom. The loom was operated by “the Middle Man,” while “Sir Archy” and the “Glove Woman” focused its rays on Matthews and “Jack the Schoolmaster” recorded their effects. Similar gangs, Matthews said, were operating looms all over London to influence the thinking of the nation’s leaders. The tortures included “fluid locking,” “cutting soul from sense,” “stone making,” “thigh talking,” and “lobster-cracking,” which Matthews also described as “sudden death-squeezing”:

In short, I do not know any better way for a person to comprehend the general nature of such lobster-cracking operation, than by supposing himself in a sufficiently large pair of nut-crackers or lobster-crackers, with teeth which should pierce as well as press him through every particle within and without; he experiencing the whole stress, torture, driving, oppressing, and crush all together.

Matthews remained an asylum inpatient until his death in 1815. His is now considered to be the first documented case of paranoid schizophrenia.

In 1987, paleontologist Tom Rich was leading a dig at Dinosaur Cove southwest of Melbourne when student Helen Wilson asked him what reward she’d get if she found a dinosaur jaw. He said he’d give her a kilo (2.2 pounds) of chocolate. She did, and he did.

Encouraged, the students asked Rich what they’d get if they found a mammal bone. These are fairly rare among dinosaur fossils in Australia, so Rich rashly promised a cubic meter of chocolate — 35 cubic feet, or about a ton.

The cove was “dug out” by 1994, and paleontologists shut down the dig. Rich sent a curious unclassified bone, perhaps a turtle humerus, to two colleagues, who recognized it as belonging to an early echidna, or spiny anteater — a mammal.

Rich now owed the students $10,000 worth of chocolate. “It turns out that it is technically impossible to make a cubic meter of chocolate, because the center would never solidify,” he told National Geographic in 2005. So he arranged for a local Cadbury factory to make a cubic meter of cocoa butter, and then turned the students loose in a room full of chocolate bars.

“It was a bit like Willy Wonka,” Wilson said. “There were chocolate bars on the counters, the tables. We carried out boxes and boxes of chocolate.”

Fittingly, the new echidna was named Kryoryctes cadburyi.

In 1962 a Swedish motorist was fined for leaving his car too long in a space with a posted time limit. The motorist objected, saying that he had removed the car in time and then happened to return to the same spot later, resetting the time limit. The policeman defended his charge, saying that he had noted the positions of the valves on two of the tires — the front-wheel valve was in the 1 o’clock position, the rear-wheel valve at 8 o’clock. If the car had been moved, he argued, the valves were unlikely to take the same positions.

The court accepted the motorist’s claim, calculating that the chance that the valves would return to the same positions by chance was 1/12 × 1/12 = 1/144, great enough to establish reasonable doubt. The court added that if all four valves had been found to be in the same position, the lower likelihood (1/12 × 1/12 × 1/12 × 1/12 = 1/20,736, it figured) would have been enough to uphold the fine.

Is this right? In evaluating this reasoning, University of Chicago law professor Hans Zeisel notes that this method is biased in favor of the defendant, since the positions of the valves are not perfectly independent. He later added, “The use of the 1/144 figure for the probability of the constable’s observations on the assumption that the defendant had driven away also can be questioned. Not only may the rotations of the tires on different axles be correlated, but the figure overlooks the observation that the car was in the same parking spot. When a person leaves a parking place, it is far from certain that the spot will be available later and that the person will choose it again. For this reason, it has been said that the probability of a coincidence is even smaller than a probability involving only the valves.”

(Hans Zeisel, “Dr. Spock and the Case of the Vanishing Women Jurors,” University of Chicago Law Review, 37:1 [Autumn 1969], 1-18)

I am watching a double solar eclipse. The heavenly body Far, traveling east, passes before the sun. Beneath it passes the smaller body Near, traveling west. Far and Near appear to be the same size from my vantage point. Which do I see?

Common sense says that I see Near, since it’s closer. But Washington University philosopher Roy Sorensen argues that in fact I see Far. Near’s existence has no effect on the pattern of light that reaches my eyes. It’s not a cause of what I’m seeing; the view would be the same without it. (Imagine, for example, that Far were much larger and Near was lost in its shadow.)

“When objects are back-lit and are seen by virtue of their silhouettes, the principles of occlusion are reversed,” Sorensen concludes. “In back-lit conditions, I can hide a small suitcase by placing a large suitcase behind it.”

See In the Dark.

(Roy Sorensen, “Seeing Intersecting Eclipses,” Journal of Philosophy XCVI, 1 (1999): 25-49.)

These tiles have a remarkable property — by working together, the four can impersonate any one of their number (click to enlarge):

The larger versions could then perform the same trick, and so on. Here’s another set:

In this set, each of the six pieces is paved by some four of them:

By the fathomlessly imaginative Lee Sallows. There’s more in his article “More on Self-Tiling Tile Sets” in last month’s issue of Mathematics Magazine.

Here’s a way to visualize multiplication that reduces it to simple counting:

Express the digits in each factor with rows of parallel lines, as shown, and then count the intersections to derive the product. This is more cumbersome than the traditional method, but its visual nature is appealing, and it permits anyone who can count to reach the right answer even if he doesn’t know the multiplication table.

The example above uses small digits, so no “carrying” is required, but the method does accommodate more complex sums — it’s explained well in this video:

See Two by Two.

(Thanks, Dieter.)