Tunnel Through Time by Lester del Rey

Go backward through space-time into prehistory? It seemed fantastic, but the machine—invented by Bob's father in connection with his research on the interrelation of time and gravity—actually exists.

Bob and his friend Pete are anxious to be the first to visit the era of the dinosaurs. Overruled, they watch Pete's father, a famous paleontologist, step into the shimmering ring—and disappear into the past. But when he does not return on schedule because the machine jams, they decide to follow him on a rescue mission...

A thrilling time-travel adventure!