There is reason to think that chains for the protection of important centres of shipping were more common than might be supposed from the few definite particulars of them which have survived. As an effective defence against the approach of the war-ships of an enemy, however, it would perhaps be impossible to find a more feeble type of protection. _Booms._--As we have already observed in dealing with chains, the necessity must have been felt of supporting such very heavy barriers, even under water and by means of buoys. The boom, although introduced quite early, must have been an improvement upon the simple iron chain, because it contained, to some extent, its own means of support. This contrivance, a chain of linked up massive timbers reinforced with iron, and armed with iron spikes was employed, as early as the time of Queen Elizabeth at Great Yarmouth, and subsequently at many other ports. Like the chain it, of course, provided an obstruction to navigation, especially at the mouths of rivers and harbours; but its massive iron spikes, calculated to pierce and damage shipping, gave it a distinct advantage over the chain. _Fire-ships_.--These were ships filled with combustibles and explosives sent to drift among the shipping of the enemy. In the action off Gravelines, fire-ships were used with considerable moral effect against the remains of the Spanish Armada, and they materially assisted in breaking up the sea-power of the Spaniards. Seven vessels were charged with combustibles and primed with gunpowder. As they neared the Spanish ships their appearance created panic. The Spaniards, in order to avoid the danger of fire, cut their ships adrift, and serious damage was caused by the collisions which ensued. In 1667, again, fire-ships were employed in the daring raid made by the Dutch in the Thames and Medway. This time they were used by the Dutch near the chain at Gillingham Reach. _Catamarans._--Another method of firing an enemy's shipping was by means of a kind of raft charged with combustibles. The idea of the Catamaran, as regards both its name and construction, was borrowed from the coasts of India and Ceylon where a raft made of three long timbers lashed together, the middle timber being the longest, is used for fishing purpose. As adapted for destroying shipping the Catamaran may be described as a kind of floating mine. Catamarans were much favoured by Mr. Pitt, and in 1804 they were employed by the English against the French fleet, but they proved unsuccessful. THE COASTGUARD The coastguard force is of great antiquity, although it is not known at what period it was instituted. In 1403-4 (5 Henry IV, c. 3) it was enacted by statute