Tunnel Through Time — 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!