Here was a possibility which had been overlooked.

How was he to instil belief that he spoke the truth?
A moment passed.

Thalassa cast another black look at him, and turned as if to walk away.

"I'll keep my word," he muttered to himself.
The young man's quick ear caught the whispered sentence, and saw the way.
"I'll prove it to you," he said.

"You promised Sisily that you'd tell nobody she was at Flint House to see her father on the night he was killed.