By the summer of 1804, Boulogne had become the assembly point of the largest invasion army Europe had seen in centuries. Napoleon's Armee de l'Angleterre - more than 100,000 men - was encamped on the heights above the port, drilling for a Channel crossing that was supposed to settle the British problem in one stroke. Napoleon himself visited on 16 August 1804 to review the troops and pin medals. He had memorably declared that his fleet need only be "masters of the Channel for six hours, and we shall be masters of the world." The invasion barges sat in Boulogne's outer roads, defended by a double line of warships anchored nose-to-tail under the guns of cliff-top batteries. British coastal defences were unready and outnumbered. The Royal Navy blockaded everything they could see. Pitt's First Lord of the Admiralty, Lord Melville, was short of ships. Something had to give.

Robert Fulton was a Pennsylvania-born artist turned engineer who had spent the late 1790s in France working on submarine warfare. His prototype submarine, Nautilus, had operated successfully in the Seine and at Brest, diving with three men aboard. The French Navy refused to commission it - in part because submarine warfare was widely considered dishonourable, in part because the French establishment did not know what to do with this strange American. Fulton crossed the Channel and offered his services to Britain. Pitt met him on 20 July 1804 alongside the naval officer Sir Home Riggs Popham. Fulton proposed an attack on Boulogne combining fireships, torpedoes (his name for moored contact mines), and floating gunpowder casks. Pitt signed a contract. Fulton's pay: £200 a month plus £40,000 for the first decked French ship destroyed, with half the value of any other ship to follow.

Fulton's "torpedoes" were not the self-propelled weapons the word later came to mean. They were copper-clad gunpowder containers, designed to be towed into position by a small boat under cover of darkness. The crew would approach an enemy ship at anchor, hook the device to the anchor cable, pull a pin to activate a clockwork timer, slip off the paddles, and row away before it detonated. The crews would also release "carcass" casks - floating barrels of gunpowder, ballast and combustible material - which would drift on the tide until they struck a hull and exploded. Captain Edward Owen of HMS Immortalite was sent to survey the French coast. In Britain, Fulton's devices were built in secret, then loaded aboard the ships of the attack force. The Royal Navy had been at sea for seven hundred years. Nothing like this had been tried.

At nine in the evening on 2 October, with wind and tide judged right, the British flotilla began its approach to the harbour. The French had not been idle. They had pulled their line of frigates further inshore, anchored a screen of pinnaces (oared armed boats) outside them, and posted sentries. The British boats slipped in under the dark coast. The sentries spotted them. The shore batteries opened fire. In the confusion that followed, Fulton's machines were deployed - clockwork timers set, paddles dropped, oars pulling the crews back into the Channel. Casks bumped against hulls; some detonated, some did not. Boulogne harbour flashed and thumped through the night. When morning came, the French fleet was essentially intact. A handful of small craft had been damaged. Fulton's first combat trial of naval mines had produced approximately no strategic result.

Fulton blamed his crews, claiming the devices had not been used properly. He refined them. Admiral Keith, who had been sceptical before the raid, was sympathetic; he thought another attempt might succeed. Winter weather closed the season. Fulton tried again at Boulogne on 19 September 1805, this time alongside Sir Sidney Smith, with similar disappointing results. By the next summer Napoleon had marched the Armee de l'Angleterre away from Boulogne to fight the Austrians at Austerlitz, and the invasion threat - whether or not it had ever been real - was over. Fulton sailed for home. In August 1807 he launched the North River Steamboat on the Hudson, the first commercially successful steamboat, and the world found out that the inventor of the underwater explosive was also the man who had finally made steam navigation pay. The Royal Navy's experiment with mine warfare went into the archive. The next naval mines used in earnest would not appear until the Crimean War, half a century later - laid, oddly, by the Russians, against the very Royal Navy that had first dared to try them at Boulogne.