Merthyr-Tydvil or Tydfil (so called from the martyrdom here of a Welsh princess of that name), a parliamentary borough and market-town of South Wales, on the confines of the counties of Glamorgan and Brecknock, 24 miles N. by W. of Cardiff, its port, and 178 W. of London. Pop. (1801) 7705; (1871) 51,949; (1901) 69,228. Surrounded by lofty and bleak hills, the town stands on the banks of the Taff, and is partly built on slag foundations, the refuse of mines in the vicinity. Its streets are narrow and irregular, but since the formation of a Local Board of Health in 1850, great improvements have been effected in the widening of thoroughfares, the supply of pure water, and the construction of effective sewage-works. The industries depend on the numerous collieries and iron and steel works in the vicinity; Merthyr being the centre of the Glamorganshire coalfield. With Aberdare it is noted for the excellence of its steam coal, and the quantity of iron and steel annually turned out from the great works of Dowlais, Cyfarthfa, and Plymouth is enormous. In 1816, and again in 1831, the town was the scene of severe riots, the latter disturbance being only quelled by the military with a loss of twenty-three lives. The parl, borough (1867), which embraces Aberdare and two other outlying districts, and in 1901 had a pop. of 122,545, returns two members.