In the third week of November, in the year 1895, a dense yellow fog settled down upon London. From the Monday to the Thursday I doubt whether it was ever possible from our windows in Baker Street to see the loom of the opposite houses.
Ñfrom "The Bruce Partington Plans"

Watson does not exaggerate; in the worst London fogs, it was impossible to see past your nose

Inhabitants of London had more to fear from their city than an unhealthy environment. Barely thirty years before Doyle's birth, London was a criminal's paradise. Whole areas of the city were "owned" by criminal groups, and honest citizens hardly dared to walk through certain neighborhoods at night, even armed. In 1829, the Metropolitan Police was founded by Home Secretary Sir Robert Peel (hence the nickname "Bobbies"). By Conan Doyle's birth in 1859, there were over 200 police constabulary units in England and Wales, under the jurisdiction of individual counties. As Conan Doyle represents them in the Sherlock Holmes stories, the constabularies were highly bureaucratized organizations that did things "by the book"; constables themselves tended to be seen as good-intentioned, but plodding, and not always successful. Luckily, unlike Inspectors Lestrade and Hopkins, Holmes's erstwhile colleagues, the Metropolitan Police was not actually forced to match wits with Holmes's brilliant nemesis, Moriarty, leader of the most insidious criminal syndicate in England.