England and Scotland were leading centres of the Scientific Revolution from the 17th century. The United Kingdom led the Industrial Revolution from the 18th century, and has continued to produce scientists and engineers credited with important advances. Major theorists from the 17th and 18th centuries include Isaac Newton, whose laws of motion and illumination of gravity have been seen as a keystone of modern science; from the 19th century Charles Darwin, whose theory of evolution by natural selection was fundamental to the development of modern biology, and James Clerk Maxwell, who formulated classical electromagnetic theory; and more recently Stephen Hawking, who advanced major theories in the fields of cosmology, quantum gravity and the investigation of black holes. Major scientific discoveries from the 18th century include hydrogen by Henry Cavendish; from the 20th century penicillin by Alexander Fleming, and the structure of DNA, by Francis Crick and others. Famous British engineers and inventors of the Industrial Revolution include James Watt, George Stephenson, Richard Arkwright, Robert Stephenson and Isambard Kingdom Brunel. Other major engineering projects and applications by people from the UK include the steam locomotive, developed by Richard Trevithick and Andrew Vivian; from the 19th century the electric motor by Michael Faraday, the first computer designed by Charles Babbage, the first commercial electrical telegraph by William Fothergill Cooke and Charles Wheatstone, the incandescent light bulb by Joseph Swan, and the first practical telephone, patented by Alexander Graham Bell; and in the 20th century the world's first working television system by John Logie Baird and others, the jet engine by Frank Whittle, the basis of the modern computer by Alan Turing, and the World Wide Web by Tim Berners-Lee. Scientific research and development remains important in British universities, with many establishing science parks to facilitate production and co-operation with industry. Between 2004 and 2008 the UK produced 7 per cent of the world's scientific research papers and had an 8 per cent share of scientific citations, the third and second-highest in the world (after the United States and China, respectively). Scientific journals produced in the UK include "Nature", the "British Medical Journal" and "The Lancet". A radial road network totals of main roads, of motorways and of paved roads. The M25, encircling London, is the largest and busiest bypass in the world. In 2009 there were a total of 34 million licensed vehicles in Great Britain. The UK has a railway network of in Great Britain and in Northern Ireland.