The _Pallas_, an armour-plated six-gun ship, and the _Research_ were given recessed ports, in order to increase their firing range, but the ports were constructed angularly and did not allow the guns to be sufficiently depressed to hit a small boat close at hand; thus the weapons would have been no defence against a torpedo-boat attack, if the latter got to close quarters. This fault was remedied in the Venezuelan transport and cruiser _Bolivar_, in which the recessed ports were fitted under the personal superintendence of their inventor, Captain Symonds, and were slanted outside the gun ports so that they would allow of a gun being depressed to strike a small boat lying nearly alongside, while their wall was curved instead of flat as in the British ships mentioned. Her sister ship, so far as dimensions were concerned, the _Mary_, was devoted to the more peaceful requirements of the cattle-trade between London and Gothenburg. The _Bolivar_ was a twin-screw vessel, and it is curious to note that even then, when this method of propulsion had proved its superiority, it was gravely stated concerning her trials that “To keep time in all weathers and in all seasons nothing is superior to the paddle, but in long voyages, especially where sails are occasionally used, the screw may be employed with advantage.”[44] Several vessels followed in rapid succession after the turret ships, and an upper deck battery was added in the _Sultan_, a vessel which otherwise much resembled the _Hercules_, and then followed a class to which the _Iron Duke_ and _Vanguard_ belonged. The conditions imposed in the construction of this class were that they should draw 22 feet 6 inches of water, that they should carry armour not less than 8 inches thick at the water-line and 6 inches elsewhere except at the bow and stern, that they should have a speed of thirteen and a half knots, and that their guns should be capable of firing in any direction. This class was named after the _Audacious_, and proved fairly successful. The principal event which distinguished the class was the accidental ramming of the _Vanguard_ by the _Iron Duke_, in September, 1875, off the coast of Ireland. They were all broadside ships, and the type was brought much nearer to perfection in the _Alexandra_, which was then the largest masted ironclad that had ever been designed; and though she was a central battery ship, four of her twelve guns could fire right ahead, and two right astern, and four to six guns could fire one either broadside. She carried two Woolwich rifled muzzle-loading guns of 25 tons each, and ten 18-ton guns. The two big guns were placed in the upper deck battery forward. As a further protection, besides her armour, the main deck battery between decks was divided in two by an armoured bulkhead. She was the first cruising armoured broadside ship in the British Navy to have engines on the compound system, and her twin screws were each driven by an independent set of engines with an