They learn how to control the machine and jump ahead into different eras, looking for one that's hospitable. Not an easy job, and Melton explores various possible futures, most of them not pleasant. Jerry and Lil are nothing if not resourceful, and they become legendary figures as they deal with slavery, spaceships, and more.
What's unique about this book is the relationship between Jerry and Lil, two teens in love but with strong moral restraints against consummating that love without the benefit of marriage. You're not going to find this kind of thing in the usual YA novel.
The Copper Room is another winner from Melton. It's an entertaining ride into several possible futures, with some nice little twists along the way. There's humor, too, and the idea of a time traveler who often wears a cheerleader outfit is especially amusing. Check it out.