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.)

A card trick by Mark Wilson:

Put your finger on any red card. Move it left or right to the nearest black card. Move it up or down to the nearest red card. Move it diagonally to the nearest black card. Now move it down or to the right to the nearest red card.

You’ll always land on the ace of diamonds.

(From Harold R. Jacobs, Mathematics: A Human Endeavor, 1970.)