Thus, putting the case mathematically, Mr. Copperdock was the highest common factor of the terms representing the four dead men. Inspector Whyland had spent much time and care in investigating their histories and their actions, and he could find no other factor common to them all. They had been, so far as he had been able to ascertain, complete strangers to one another, and they could never consciously have met one another. The only characteristics which they had in common were that they had all received counters numbered in the order of their deaths, that they were all males, and that their ages had all been over fifty. Naturally, the mind of Praed Street was not greatly relieved by the verdict on the death of Mr. Martin. Violent death was becoming far too frequent an incident for any of the local community to feel safe. Men wondered when it would be their turn to find the fatal counter, and how they should ward off the death which so swiftly followed. It was not as though one could tell exactly what one had to guard against. The knife had been used twice, certainly, but poison in one form or another had accounted for two of the deaths. The numbered counters were the characteristic feature of the case which appealed most strongly to the popular imagination. Inexplicable murders were common enough, even murders in which no motive or criminal had ever been found. But a series of murders, each prefixed by a definite warning, were an entirely novel sensation. The theory of the counters was publicly discussed from every possible point of view, without any really satisfactory theory being arrived at. Quite a large proportion of the population of London opened their morning papers in expectation of finding news of the delivery of counter number V. The news, when it came, illustrated the hold which the subject had upon men’s minds, and the moral effect which it has produced. Mr. Ludgrove read the story as he consumed his frugal breakfast, and he was not surprised to hear Mr. Copperdock’s voice in the shop a few minutes later. “Come in, Mr. Copperdock,” he called, and the tobacconist, paper in hand, the light of excitement in his eyes, entered through the curtained door. “Have you seen this about this old chap Goodwin?” he enquired, without preliminary. “I have indeed,” replied Mr. Ludgrove gravely. “I imagine that the whole thing was an utterly heartless practical joke. I only hope the police will be able to trace the sender.”