Once he stopped, and, retracing half a step, called out a facetious after-thought.

The boys grunted impatiently, and the blacksmith swore in his beard.

Then the soldier took another step back, laughing at his wit, yet moving irresolutely, as though he had another word or two to add to the joke.
After this his progress backwards was steady.
At last, when he was within a yard of the precise spot, and not one of the attacking party had a grain of patience left, the smith dropped his hand, and Jack toppled the stone over the edge.

It fell with a terrible swiftness; the soldier completed his yard of step, and the block took him, not on the crown, but on the right shoulder.

It was, however, enough.