The valuable, if not indispensable, nature of the services which may be rendered by Special Constables when directed towards the suppression of riots, already indicated at Birmingham in 1839, was more fully exemplified in 1848, when the Chartists assembled at Kennington Common, threatened to descend upon London. The fear that the Metropolis might be subjected to an access of anarchy similar to that which bestowed an evil notoriety on the Gordon Riots, gave rise to preparations for defence on an unprecedented scale. At this crisis not less than two hundred thousand citizens enrolled themselves as Special Constables; the Metropolitan Police were told off to guard the bridges; and all available troops were held in reserve, mostly in houses on the Middlesex side of the river. The Duke of Wellington took command of this huge police army, and so perfect were his arrangements that, without the display of a single redcoat, the rioters outside dared not advance to the attack, and their sympathisers within dared not make any diversion in their favour. The whole subject of the policing of riotous and riotously disposed crowds is of the highest importance; but being a science in itself, its many aspects cannot be discussed in a book which only professes to tell briefly the story of police development in this country. At the same time, if we are to appreciate correctly the bearing of one fact upon another, if we hope to draw intelligent conclusions, it is impossible to avoid altogether what may be called the semi-scientific side of the question. In order to estimate the efficiency of any police force, to decide, for example, whether its failures are due to faulty organization or to bad generalship,--it is not sufficient to have an acquaintance with a mass of facts, dates and figures, but a key is required to read the cipher, some guide is necessary to assist in arranging the known factors in the order of their relative importance. An indifferently directed force may be capable of dealing in detail with isolated offenders; but the problem of pacifying a combination of peace breakers may induce a strain which will find out the faulty link in all but the most highly tempered organizations. Just as a concerted breach of the peace by a number of persons is disproportionately more serious than an independent breach by any individual, so also do police duties take a higher range as soon as it becomes necessary to concentrate the energies of a number of constables for the attainment of a common object. If he is to employ his constables to the best advantage, a superior officer of police must have some knowledge of the general laws that sway men when they are assembled together in great numbers, those "volcanic forces" which, in the words of an American writer, "lie smouldering in all ignorant masses."