The British Empire didn’t merely help perfect the modern timepiece but also helped to popularize the watch. In the late 1800s, watches were considered to be feminine jewelry; men kept their timepieces tucked away in pockets. But in colonial campaigns like the First Boer War and the Third Burmese War, British commanders tied little clocks to their soldiers’ wrists. Going into battle with feminine jewelry might have struck the men of war as uniform malfunction. But the innovation proved extremely useful for coordinating troop movements.

By World War I, watches were standard-issue gear for soldiers in the trenches. When the men who survived came home, they retained the habit. Thus the wristwatch, conceived as a piece of jewelry for women, was re-marketed through colonial warfare as a thoroughly masculine fashion. By the 1930s, wristwatches were the norm and the pocket watch was an anachronism. Time, itself, had become a human appendage.

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2. Time-Zone Travel

Time and space are connected, not only in the fabric of the universe, but also in our idioms. We talk about time as an interval applying both to moments (“It’s fifteen minutes to five”) and to geography (“I’m fifteen minutes from Five Guys”). Perhaps this is why the invention of a machine to zoom through space, the train, inspired the idea that a machine might travel through time.

The rise of the railroad in the 1800s startled the era’s scientists and inspired a new ecstatic language of progress. In 1864’s Journey to the Center of the Earth, Jules Verne imagined a machine that, rather than navigate the circumference of the earth, departed along the perpendicular axis to travel inward through the mantle of its sphere. In 1895’s The Time Machine, H.G. Wells’ protagonist embarks along another dimension, time, as if history itself were a navigable rail line stretching from past to future. Humans had been trying to predict the future since before the Oracle of Delphi. Only after the invention of trains did they imagine visiting it.

The discovery of machine-power was, in many ways, the discovery of the future. “Travelers riding in steam-driven railroad trains looked out their windows onto a landscape where oxen plowed the fields as they had done in medieval times, horses still hauled and harrowed, yet telegraph wires split the sky,” James Gleick writes in Time Travel, his wondrous interdisciplinary history of the subject. (Those interested in a simpler history of time might also enjoy the delightful young-adult book This Book Is About Time.) “This caused a new kind of confusion or dissociation,” Gleick wrote. “Call it temporal dissonance.”

Dissonance is right. The railroad created a crisis of time management unlike anything human beings had ever experienced. In the pre-train age, all time was local, divined mostly by the angle of the sun in the sky. If Philadelphia and Harrisburg had different times, nobody noticed, because a Philly resident couldn’t reach Harrisburg by phone or rail to tell the difference. As a result there were hundreds of local times in the United States.