Free Zoom: From Atoms and Galaxies to Blizzards and Bees: How Everything Moves by Bob Berman Page B

away.
    Our final desert-motion phenom is its most renowned specialty: the mirage. As we all know, mirages are common on hot surfaces, such as a highway on a summer afternoon. The culprit is the changing speed of light. Despite its reputation as a constant, light travels more slowly through cool air. But the hot air above a summer road or broiling sand lets light move faster right there, closer to its vacuum speed, and this change bends, or refracts, images hitting it. The result is a mirror effect. The air reflects the sky, perfectly mimicking a puddle of water.
    But finding any movement was an impossible job when I was in the desert. Nothing budged once those dust devils died. The absence of flowing water, moving clouds, circling birds, buzzing insects, or rustling leaves makes the desert visually frozen. A still photograph. Its landscape offers the antithesis of animation.
    But later there came a few hot afternoon gusts. Bits of sand blew momentarily. The still life came alive. Clearly the dunes migrate over time. And when it comes to shifting sands, only one person is associated with their vagaries. British brigadier Ralph Alger Bagnold.
    He was the archetypical English stiff-upper-lip, military-cum-Renaissance man. Bagnold was born in 1896, son of a derring-do colonel in the Royal Engineers who gloriously participated in the 1884–85 rescue expedition that attempted to free Major General Charles George Gordon from Khartoum. His sister was Enid Bagnold, who wrote the bestselling 1935 novel National Velvet.
    Armed with this odd genetic pedigree, Bagnold attended Malvern College, joined the Royal Engineers, as his dad did, and received medals for serving in the miserable World War I French trenches for three years. After the war Bagnold studied engineering and earned a master of arts degree at Cambridge University. He returned to active duty in 1921 and then got swept into his lifelong calling. He served in Cairo and the Thar wastelands of northwestern India, and at both places he spent every spare minute exploring the desert.
    Bagnold described his extensive excursions in his book Libyan Sands: Travel in a Dead World (1935). He developed a special type of compass that would not go awry around the iron ore often buried in arid regions. It was he who discovered that one really could drive a car across the Sahara as long as you let most of the air out of the tires and kept punching the gas pedal when the sands got deep. You got the feeling this was knowledge gained the hard way.
    Although a third of the world’s deserts are covered with sand, there has been very little research into these ergs, as sand-covered desert areas are, oddly, called—probably because it’s hard to travel there or even reach many of them, and at that point it’s very slow going to make much physical progress. Bagnold changed that with his still-definitive book, published in 1941, The Physics of Blown Sand and Desert Dunes, which is every bit as tedious as it sounds. After the first two or three chapters I discovered that it is not a page-turner, despite the Amazon five-star rating that lured me to purchase it. But no one to this day has improved on its revelations. Bagnold used wind-tunnel experiments to predict sand movement and confirmed these expectations with extensive observations in the Libyan desert.
    Basically, sand is characterized by its size rather than its composition. Bagnold defines sand as any particle between 0.02 millimeter and 1.0 millimeter in diameter, although later experts generously expanded the upper range by more than 50 percent, to 1.6 millimeters—a fifteenth of an inch. Size matters, because sand is defined as consisting of grains small enough to be moved by the wind but too heavy to remain in suspension in the air, as dust and silt do. Particles too heavy to be blown by wind are classified as pebbles or gravel. If it’s smaller than a thousandth of a millimeter, a particle essentially remains suspended in the