The third cause is the acquisition by the flotilla of battle power. It is a feature of naval warfare that is entirely new.[10] For all practical purposes it was unknown until the full development of the mobile torpedo. It is true that the fireship as originally conceived was regarded as having something of the same power. During the Dutch wars--the heyday of its vogue--its assigned power was on some occasions actually realised, as in the burning of Lord Sandwich's flagship at the battle of Solebay, and the destruction of the Spanish-Dutch fleet at Palermo by Duquesne. But as the "nimbleness" of great-ships increased with the ripening of seamanship and naval architecture, the fireship as a battle weapon became almost negligible, while a fleet at anchor was found to be thoroughly defensible by its own picket-boats. Towards the middle of the eighteenth century indeed the occasions on which the fireship could be used for its special purpose was regarded as highly exceptional, and though the type was retained till the end of the century, its normal functions differed not at all from those of the rest of the flotilla of which it then formed part. [10] But not without analogous precedent. In the later Middle Ages small craft were assigned the function in battle of trying to wedge up the rudders of great ships or bore holes between wind and water. See Fighting Instructions (Navy Record Society), p. 13. Those functions, as we have seen, expressed the cruising idea in its purest sense. It was numbers and mobility that determined flotilla types rather than armament or capacity for sea-endurance. Their primary purpose was to control communications in home and colonial waters against weakly armed privateers. The type which these duties determined fitted them adequately for the secondary purpose of inshore and despatch work with a fleet. It was, moreover, on the ubiquity which their numbers gave them, and on their power of dealing with unarmed or lightly armed vessels, that we relied for our first line of defence against invasion. These latter duties were of course exceptional, and the Navy List did not carry as a rule sufficient numbers for the purpose. But a special value of the class was that it was capable of rapid and almost indefinite expansion from the mercantile marine. Anything that could carry a gun had its use, and during the period of the Napoleonic threat the defence flotilla rose all told to considerably over a thousand units.