In the hound of the baskervilles how does Doyle us the metaphor of three broken thread ?

Doyle uses the three threads as a metaphor for the clues they have in the investigation. The first clue was Barrymore's location. The second clue was Cartwright's failure to find the newspaper, and the last is Clayton's inability to provide a name. These threads were the clues that needed to be followed...... when the thread breaks.... Holmes must return to the beginning.

“Snap goes our third thread, and we end where we began”