Midsummer Century

When John Martels' new radio telescope obstinately refuses to function, he decides to see for himself what the trouble is. Field strength detector in hand, he clambers up the side of the wave guide -- and falls straight down the tube. The fall is bad enough to put him in hospital for weeks. But it doesn't. In fact, when Martels regains consciousness, it takes him a little time to grasp exactly where he is: trapped inside someone else's brain, about 23,000 years in the future...