There were three principal reasons why the immediate provision of a general scheme of police reform in the Boroughs was urgently required at this particular moment. Criminals of all kinds are ever on the alert to find an Alsatia where they can ply their trades without the unwelcome interference of their hereditary enemy, the policeman, and the formation of the Metropolitan force was the signal for a wholesale exodus of depredators from London towards other and more secluded centres of activity. The smaller country towns and rural districts offered few attractions to enterprising thieves, and so boroughs like Liverpool, Manchester and Bristol, where considerable plunder was to be had at little risk, were the chosen retreats of many who no longer dared to brave it out in London. If any reliance may be placed on contemporary statistics, this migration entirely altered the distribution of the criminal classes: it was estimated, incredible as it may seem, that in 1797 nearly ten per cent. of the population of London supported themselves "by pursuits either criminally illegal or immoral":[196] in 1837, the proportion of known bad characters to the population was calculated at only 1 to 89 in the Metropolitan Police District, but at 1 to 45 in the Borough of Liverpool, 1 to 31 in the City of Bristol, and 1 to 27 in the Town of Newcastle-on-Tyne.[197] One cannot believe that these calculations were based on perfectly accurate information, but on the other hand it is unlikely that the results are so wide of the mark as to be altogether without value, and after allowing a large margin for error, proof enough remains that the enhanced security of London had, to some extent, been purchased at the expense of the inhabitants of other towns. The desire to acquire wealth without working for it--to "live idly yet to fare well"--is the main incentive that makes men criminal, and experience proves that if those who live by their wits are sufficiently harassed, they soon show their wit by returning to the humdrum path of honesty till such time as the vigilance of their enemies is relaxed. "Allow the thief no rest" is a sound maxim of preventive police; and if in 1829 it had been possible simultaneously to provide efficient police for all the towns of England, many and many a thief would have been driven to exchange his old occupation for some less precarious trade. The second reason why delay was dangerous was because of the enormous increase of population in the manufacturing towns. Trade was brisk, money plentiful, and the rapid development of the railway system caused an increasing stream of workers to flow from the country into the industrial centres. The population of Birmingham, for instance, increased in this way from 90,000 in 1815 to 150,000 in 1832.