Plumber's smoke rocket

"It is nothing very formidable," he said, taking a long cigar- shaped roll from his pocket. "It is an ordinary plumber's smoke- rocket, fitted with a cap at either end to make it self-lighting. Your task is confined to that. When you raise your cry of fire, it will be taken up by quite a number of people. You may then walk to the end of the street, and I will rejoin you in ten minutes. I hope that I have made myself clear?"

(Aventure I, A SCANDAL IN BOHEMIA pg. 13)

Before the era of pressure testing, in the Victorian Era, plumbers used a smoke rocket to detect leaks in pipes. The smoking device was inserted into a pipe and lit up. Smoke would come out from whichever leaks there were in the pipe.