In "Funny Ha-Ha, Funny Strange, and Other Reactions to Incongruity," John Morreall discusses this classic Addams cartoon as an example of a visual joke. No doubt he's right: it's funny. But it's also a great illustration of the close relationship between magic and comedy.
Imagine the cartoon without the uphill skier looking over his shoulder. No longer quite so funny? Why not? The answer, I think, is that our effort to imagine what the uphill skier might have witnessed—an effort that ends in failure—is a big part of our amusement. And it's here that there's a close relationship to magic.
Suppose that David Blaine is skiing downhill toward a large tree in untracked powder. Just as Blaine approaches the tree—which also stands in untracked snow—a simple black curtain drops and obscures his passage. A moment later, the curtain is snapped back and Blaine, still skiing, appears to have left tracks that go around the tree, just as in Addams' drawing. The result is a performance that, like Addams' cartoon, all but forces you to try to imagine: how? What would you see if you could, like the uphill skier in the cartoon, look behind the curtain?
Addams' classic cartoon is like a single-frame picture of a complete magic trick. It amuses us precisely because it leaves the mind in a specific sort of tailspin—just like good magic.
Shepard-Risset Glissando (click to play!)
Risset Rhythm (click to play!)
A descending Shepard-Risset Glissando seems to get lower but at the same time seems to remain constant in pitch. Since no sound can change in pitch and at the same time remain constant in pitch, you experience an apparently impossible sound. (Compare a color that—impossibly—looks both red and green.)
Similarly, an accelerating Risset Rhythm (based on similar principles) seems to speed up but at the same time seems to have a constant tempo. It therefore constitutes an apparently impossible rhythm.
To maximize your auditory bewilderment, play both tracks simultaneously.
Here is the question that interests me (it has received almost no attention from philosophers): Why do we find these sorts of experiences engaging? (I hesitate to say that we find them pleasurable. Some people do not—just as some people don't like magic, or philosophy!) This is one of the motivating questions of my project, Antinomic Aesthetics.
For a musical composition that incorporates the Shepard-Risset Glissando, see Jean-Claude Rissett's "Computer Suite from Little Boy" (1968).
Finally, for more information about Shepard Tones and the Shepard-Risset Glissando, see this Wikipedia page.