Portsmouth was the largest industrial site in the world; it had a workforce of 8,000, and an annual budget of £570,000. In 1805, Admiral Nelson left Portsmouth to command the fleet which defeated France and Spain at the Battle of Trafalgar. The Royal Navy's reliance on Portsmouth led to its becoming the most fortified city in the world. The Royal Navy's West Africa Squadron, tasked with halting the slave trade, began operating out of Portsmouth in 1808. A network of forts, known as the Palmerston Forts, was built around the town as part of a programme led by Prime Minister Lord Palmerston to defend British military bases from an inland attack following an Anglo-French war scare in 1859. The forts were nicknamed "Palmerston's Follies" because their armaments were pointed inland and not out to sea. In April 1811, the Portsea Island Company constructed the first piped-water supply to upper- and middle-class houses. It supplied water to about 4,500 of Portsmouth's 14,000 houses, generating an income of £5,000 a year. HMS Victorys active career ended in 1812, when she was moored in Portsmouth Harbour and used as a depot ship. The town of Gosport contributed £75 a year to the ship's maintenance. In 1818, John Pounds began teaching working-class children in the country's first ragged school. The Portsea Improvement Commissioners installed gas street lighting throughout Portsmouth in 1820, followed by Old Portsmouth three years later. During the 19th century, Portsmouth expanded across Portsea Island. Buckland was merged into the town by the 1860s, and Fratton and Stamshaw were incorporated by the next decade. Between 1865 and 1870, the council built sewers after more than 800 people died in a cholera epidemic; according to a by-law, any house within of a sewer had to be connected to it. By 1871 the population had risen to 100,000, and the national census listed Portsmouth's population as 113,569. A working-class suburb was constructed in the 1870s, when about 1,820 houses were built, and it became Somerstown. Despite public-health improvements, 514 people died in an 1872 smallpox epidemic. On 21 December of that year, the Challenger expedition embarked on a circumnavigation of the globe for scientific research. When the British Empire was at its height of power, covering a quarter of Earth's total land area and 458 million people at the turn of the 20th century, Portsmouth was considered "the world's greatest naval port". In 1900,