The Pax Britannica (1815–1914) was enforced directly by the global supremacy of the Royal Navy and indirectly by its commercial pre-eminence. Amassing an empire over which ‘the sun never sets’ was only one aspect of this dominance. Through the imposition of imperial free trade, Britain effectively controlled the economies of nations outside its ambit, from China to Argentina. The conversion of the Navy to steam revolutionized its range and versatility, but it rendered coaling the ‘greatest logistical headache of the age’. Britain flecked the globe with secure refuelling stops, with coal reserves in their hinterlands. However, only Welsh coal met the Navy’s exacting standards (Chinese, Australian and American coal either produced too much ash or tended to self-combust) and was shipped worldwide to cut with inferior local product. The Navy evinced its power by patrolling the key nexus points of maritime trade from the Straits of Gibraltar to the Straits of Malacca.