Middlesex, a small county in the south of England, bounded on the N. by Hertfordshire, on the E. by Essex, on the W. by Buckinghamshire, and on the S. by the Thames and the county of London. The northern boundary is irregular, having been determined originally by the estates of the abbey of St Albans and the bishopric of Ely. On the east the river Lea and on the west the Colne and the Brent form the natural boundaries. Although the area is but 233 sq. mi., the population is large (560,012 in 1891), which is accounted for by the neighbourhood of the county and city of London. We first hear of Middlesex as a sub-kingdom dependent on Essex. Its position between the territory of the East Saxons and that of the West Saxons accounts for the name. The greater part of the surface was covered with a forest, of which Enfield Chase and Hampstead Heath are relics; but it was traversed by the great road which crossed the Thames, probably by a ford at Westminster, and led north-westward under the name of the Watling Street. The population was very sparse, and it has been remarked that no castle stood within its boundary, and no great abbey except that of Westminster. After the Conquest we hear little of the county until 1101, when Henry I. granted it in farm to the citizens of London. The position of Middlesex thenceforward until the passing of the Local Government Act in 1888 was wholly peculiar. For a rent of £300 per annum the citizens had the appointment of the sheriff and all other regal rights. It was usual for the sheriffs of the city to hold the office on alternate days, whence the legal form, 'the sheriffs of London and sheriff of Middlesex.' The whole body of citizens held the office, and their nominees were strictly speaking not high but sub-sheriffs, while the Lord Mayor was Lord-lieutenant. Under this régime, as is well known, the county shared in the prosperity of its great neighbour, and became at last so populous that by the act already named those portions of it which lay nearest the Thames and the city were severed from it, and, with certain districts of Kent and Surrey, were incorporated into a new 'county of London' (q.v.). At the same time a sheriff for Middlesex and a lord-lieutenant were appointed by the crown, and a singular usage which had subsisted for more than seven centuries ceased to obtain.
The geological features of Middlesex are of a simple character. It has no high hills, no great rivers, no picturesque valleys; but the low rolling undulations consist of what is known as London clay, topped here and there with river-drift, in which indications of early human life have been detected, as well as the fossil remains of elephants and other now extinct animals. There is but little tillage, except for market-gardens, and a great part of the county consists of grazing land, being occupied largely with villa residences, surrounded in many places with large parks. Brickfields occupy the western border, and the number of large suburban villages—without, however, any important town—is remarkable. Brentford, Uxbridge, and Ealing are to the west of London, and the first-named is usually reckoned the county town. Northward are Harrow, with its school, Enfield, and Tottenham. Eastward are Highgate and Hornsey. London, it may be well to note, was never in Middlesex.
Middlesex, and especially its eastern border, was the scene of many conflicts with the Danes. During the Wars of the Roses, Barnet on the northern verge gave its name to the battle on the neighbouring Hadley Common, where in 1471 the King-maker was defeated and slain. The principal mansions are Hampton Court (q.v.), Sion House (see ISLESWORTH), and Osterley, near Hounslow, which belongs to Lord Jersey, and is a handsome building by Robert Adam.