Lester Del Rey

“ Old Ned finished. "The diagrams of his machine are still in the drawers. But Pete never showed up. I tell you, keep your fool grandson out of time machines, Lem. They don't work. Too many paradoxes—if they'd work, you could steal a future invention, get credit for inventing it, and nobody would ever have to invent it. When things have that many angles that can't work, the thing itself can't work." ”

“ His fingers slipped over the pencil smoothly, as they had always done when he worked, drafting robot bodies in the old days. A rude schematic seemed to grow almost instantly on the paper.
Lem took it, then stiffened suddenly. "Who told you?"
"A youngster named Pete LeFranc—and it was forty years ... no, over fifty years ago. Lem, if you like your grandson, keep him out of the machine. Four days, four weeks—they don't mean anything. Time machines don't work, however well they seem to." ”

“ One of the robots was quietly restraining a nervous young man who was trying to break free and join the group. His face was tense, excited, with an odd bitter fear behind it. His words were seemingly cut out of steel. "... ”