Este (muito) conto é o " O Pêndulo " de Ray Bradbury, publicado originalmente em 1939.
Você pode ler o texto completo on-line aqui
Day after day the robots still came, worked, unabated by the visitation of the black horde. They came every week, brot food, tinkered, checked, oiled, cleaned. Up and down, back and forth—THE PENDULUM!
... a thousand years must have passed before the sky again showed life over the dead Earth. A silvery bullet of space dropped from the clouds, steaming, and hovered over the dead city where now only a few solitary robots performed their tasks. In the gathering dusk the lites of the metropolis glimmered on. Other automatons appeared on the rampways like spiders on twisting webs, scurrying about, checking, oiling, working in their crisp mechanical manner.
And the creatures in the alien projectile found the time mechanism, the pendulum swinging up and down, back and forth, up and down. The robots still cared for it, oiled it, tinkering.
A thousand years this pendulum had swung. Made of glass the round disk at the bottom was, but now when food was lowered by the robots through the tube it lay untouched. Later, when the vacuum tube came down and cleaned out the cell it took that very food with it.
Back and forth—up and down.
The visitors saw something inside the pendulum. Pressed closely to the glass side of the cell was the face of a whitened skull—a skeleton visage that stared out over the city with empty sockets and an enigmatical smile wreathing its lipless teeth.