Napoleon III, since the Crimean War Britain's closest ally, visited London in April 1855, and from 17 to 28 August the same year Victoria and Albert returned the visit. Napoleon III met the couple at Dunkirk and accompanied them to Paris. They visited the Exposition Universelle (a successor to Albert's 1851 brainchild the Great Exhibition) and Napoleon I's tomb at Les Invalides (to which his remains had only been returned in 1840), and were guests of honour at a 1,200-guest ball at the Palace of Versailles.
Try to answer this question if possible (otherwise reply "unanswerable"): When were Napoleon I's remains interred in his mausoleum?
1840