Four Days' Battle (c1666). Ink on canvas. 119 x 182. The Four Days' Battle was a naval battle of the Second Anglo-Dutch War. Fought from 1 June to 4 June 1666 in the Julian or Old Style calendar then used in England off the Flemish and English coast, it remains one of the longest naval engagements in history. The Dutch accounts refer to its dates as 11 June to 14 June 1666 using the New Style calendar. The Dutch inflicted significant damage on the English fleet which lost ten ships in total, with over 1,000 men killed including two vice-admirals, Sir Christopher Myngs and Sir William Berkeley, while about 2000 English were taken prisoner including a third vice-admiral George Ayscue. Dutch losses were four ships destroyed by fire and over 1,550 men killed, including Lieutent Admiral Cornelis Evertsen, Vice Admiral Abraham van der Hulst and Rear Admiral Frederik Stachouwer. Although the result was a clear Dutch victory, it did not render the English fleet incapable of further action, and it was able to prevent a Dutch attempt to attack and destroy it at anchor the Thames estuary in early July and, after refitting, to defeat the Dutch fleet off the North Foreland on 25 July off the in the St. James's Day Battle. The introduction of sailing ships with a square rig, of a type later called the ship of the line, which were heavily armed with cannon, brought about a gradual change in naval tactics. Before and during the First Dutch War, fleet encounters were chaotic and consisted of individual ships or squadrons of one side attacking the other, firing from either side as opportunities arose but often relying on capturing enemy ships by boarding. Ships in each squadron were supposed as their first priority to support those in the same squadron, particularly their flag officer. However, in the melee of battle, ships of the same squadron frequently blocked each other's fields of fire and collisions between them were not uncommon. Although Lieutenant-Admiral Maarten Tromp had formed a line against the Spanish fleet in 1639 in the Action of 18 September 1639, this was not a planned formation but a desperate attempt to hold off a greatly superior, but badly organised, enemy. The initial sea battles of the First Dutch War were largely indecisive melees, but later in that war Robert Blake and George Monck issued instructions for each squadron to stay in line with its flag officer. At the Battle of Portland Tromp's attempt to overwhelm the English rear by concentrating his whole fleet against it and using his favourite tactic of boarding was frustrated by it remaining in line ahead At the Battle of the Gabbard, the English fleet in line ahead forced the Dutch into an artillery duel that defeated their more lightly-armed ships with a loss of Dutch 17 ships sunk or captured. Between the first and second wars, the Dutch built larger ships with heavier armament, although the shallow waters around the Netherlands prevented them building ships as big as the largest English ones, and even English ships of the same size tended to have more and larger guns than their Dutch equivalents. At the time of the Second Dutch War, the English fleet also had a signalling system which, if still rudimentary, was better than the Dutch reliance on standing instructions to fight in line. In the Battle of Lowestoft and the St. James's Day Battle, the English fighting in line ahead defeated the Dutch who did not. De Ruyter favoured the tactic of concentrating his attack on a portion of the enemy's line and achieving a breakthrough and capture of ships by boarding. However, in the Four Days' Battle the Dutch generally fought in line and the English fleet did not at important stages in the fighting. From early in the 17th century, the Dutch navy had used fireships extensively and in the First Anglo-Dutch War at the Battle of Scheveningen, Dutch fireships burned two English warships and an English fireship burned a Dutch warship. The Dutch in particular increased the number of their fireships after the outbreak of the Second Anglo-Dutch War but, at the Battle of Lowestoft, it was two English fireships that burned six Dutch warships which had collided and become entangled with one another. However, the limitations of fireships when used in open waters was demonstrated during the Four Days' Battle, where many where destroyed while trying to attack well-armed ships able to manoeuvre freely. The surrender of the English HMS Prince Royal when attacked by several Dutch fireships after it had run aground because of the panic this attack caused only demonstrated that fireships were useful against warships that were stationary or in confined harbours, but not those able to move in the open sea.