All at once, as he watched Gianbattista's twisted attitude, as the apprentice strained himself and turned so as to drive the screw effectually, the foot of the ladder seemed to move a little on the smooth marble pavement.

With a quick movement Don Paolo stepped forward, with the intention of grasping the ladder.
Hearing the sound of rapid steps, Gianbattista turned his head and a part of his body to see what had happened.

The sudden movement shifted the weight, and definitely destroyed the balance of the ladder.

With a sharp screech, like that of a bad pencil scratching on a slate, the lower ends of the uprights slipped outward from the pillar.
Gianbattista clutched at the metal bars desperately, but the long screw-driver in his hands impeded him, and he missed his hold.
Don Paolo, the sound of whose step had at first made the young man turn, and had thus probably precipitated the accident, sprang forward, threw himself under the falling ladder, and grasped it with all his might.

But it was too late.