Prof: Have you ever heard something like this on the news: "Air Force One has landed, and the president and first lady are now walking across the tarmac"? Ever wonder what tarmac is? If so, take heart: you're not alone. Tarmac, it turns out, is a type of surface pavement that is short for [enunciates slowly and clearly] tar-penetration macadam. Got that? It's a mouthful. Actually, here in the United States we don't use Tarmac much anymore, because it's been supp — that is, upstaged, by asphalt. But we still use the term when referring to the pavement at airports.

The "macadam" part of Tarmac refers to a Scotsman named John McAdam. McAdam, in the early nineteenth century, invented a method to strengthen paved roads and increase the way water drains off roadways. He named his technique, modestly, "macadam roads" [laughter]. Macadam roads had a sloped roadway covered with three layers of grav [false start] angular gravel, which was compacted by a heavy roller. These roads were undoubtedly stronger than previous ones, but they posed a new difficulty: dust. When automobiles began using them, the cars raised swirling clouds of debris, making it, shall we say, daunting to see. So, as early as the 1830s, people started experimenting by covering macadam roads with tar and sand, in an attempt solve this problem.

In 1901, a man named E. Purnell Hooley [pause] really [laughter] invented a mixture of tar and a furnace waste material called slag, to come up with the infamous tar-penetration macadam — Tarmac — that improved the dust resistance on macadam roads. Soon, roads throughout England were resurfaced with Hooley's invention, and the company he founded, Tarmac Limited, became very profitable. During World War II, the British used Tarmac to build airstrips for jet fighters, which is why we still use the term Tarmac today as a synonym for runways and other paved airport areas. OK. Now from Britain, Tarmac spread to the United States, but Americans preferred asphalt, which is a substance that occurs naturally in lakes and rocks, as well as synthetically as a byproduct of petroleum production. Asphalt is more resilient than tar, and holds up better in a wider range of temperatures. Its first recorded use dates back to about 3,000 BC, when the Sumerians used it to, uh, preserve mummies, waterproof ships, and cement bricks. Hot-mix asphalt, or HMA, is a combination of aggregates - like sand, gravel or, um, crushed stone minerals - and an asphalt binder, like coal tar. It was first used in the US for crosswalks and sidewalks in the late 19 —er, 1860s. The first road was paved with HMA in 1870, in New Jersey.

Until about 1900, US producers used natural asphalt, which they got mainly from two large lakes in Venezuela. Refined petroleum asphalt first appeared in the 1870s, and by 1907 its production out-paced natural asphalt. HMA pavement took its modern form in the early twentieth century, when Frederick Warren earned patents for a hot-mix asphalt paving that he termed "bitulithic." That's B-I-T-U-L-I-T-H-I-C. Typically, bitulithic mix contains ingredients that make it more "fluid" than sheet asphalt. Laura Ingalls Wilder, a noted American author, described this fluidity upon her first encounter with asphalt, as she watched ladies walk across the asphalt pavement. "Their heels dented the street, and while we watched, these dents slowly filled up and smoothed themselves out," she wrote. "It was like magic."

Warren's patents expired in 1920, and since then asphalt mixes have improved. As the popularity of motor vehicles skyrocketed, local governments scrambled to construct more and better roads, leading to technological innovations in both asphalt production and spreading techniques. Today, HMA covers almost 95 percent of America's paved roadways, as well as most of its sidewalks, runways, driveways, parking lots and tennis courts.