THE HISTORY LESSON

Although Professor Michael Watson had won many awards for teaching the history of psychology, he had an advantage over his colleagues: He had persuaded his department head to purchase a time machine for class excursions. Of course, the limited funds only enabled Watson to purchase the least-expensive model of Stoelting’s Waybach machine, but it got the job done.

When anyone accused Professor Watson of having an unfair advantage in teaching his class, he would patiently explain, “You have to know where and when to go with the Waybach. You can’t just go willy nilly into the past. If you do that, you’re liable to change the future, I mean the present, and when you come back, you’ll find you were never born. And then where would you be?”

Where indeed? But Watson had read the instructions for his Waybach carefully, and he knew that time travel was generally quite safe. According to the booklet, the craft—which reminded him of an antique school bus without wheels—and its occupants should always stay a few seconds earlier in time than the moment they were visiting. This slight difference meant that Watson, his class, and the machine, as part of the period’s past, were invisible to anyone and anything in the selected period.

Of course, in order to remain invisible, the craft had to stay in the past. The instructions had been very clear on the point that if something happened to the time selector—if someone accidentally nudged it, for example—and the machine and its occupants became synchronized in time with the visited period, they would suddenly become visible to whomever they were visiting. To prevent such an accident from occurring, more expensive and newer models of the Waybach made it difficult to change the time setting accidentally—or intentionally, for that matter.