From farther away I also heard fainter and lower tappings.

Continually these tappings were interrupted by the snarling of the guard.

On occasion, when the tapping went on too persistently, extra guards were summoned, and I knew by the sounds that men were being strait-jacketed.
The matter was easy of explanation.

I had known, as every prisoner in San Quentin knew, that the two men in solitary were Ed Morrell and Jake Oppenheimer.

And I knew that these were the two men who tapped knuckle- talk to each other and were punished for so doing.
That the code they used was simple I had not the slightest doubt, yet I devoted many hours to a vain effort to work it out.