Suddenly, without warning of any sort, he put out one arm and threw Clancy sideways, so that he fell over a heap of crushed stone.

Another moment and Porter had leaped for a flight of stairs and had vanished downward into the body of the mill.
It was all so quickly done that Frank was taken by surprise.

The thought flashed through his mind that Porter, unless he knew something about Professor Borrodaile and suspected why the boys were there, would not be showing his teeth in that fashion.

An instant after the prospector had disappeared down the stairs, Frank jumped after him.

Ballard followed close on Frank's heels; and Clancy, hastily picking himself up, stifled an exclamation of anger and rushed after Ballard.
The stairs led down to the floor where the boxes were placed, and where the plates, whose silver recovered the gold from the ore, stretched the length of the mill.