In the late 'nineties it was assumed that quick-firing artillery was best suited to the work of a cruiser, and thus the 6-in. gun was exclusively adopted. But since then Naval strategists have developed their ideas as to the function of armoured cruisers, and now anticipate their use in the line of battle; so that not only has the defensive quality been improved, but the offensive power has been materially increased. In the _Defence_, and the other ships of the class, the 6-in. gun has been entirely discarded in favour of an installation of 9.2-in. and 7.5-in. weapons. Owing to the perfection of the hydraulic and electric mountings, little has been forfeited in respect of rapidity of fire, while much has been gained in the striking energy at a given range of each projectile. Thus, while the 6-in. gun five years ago had an energy equal to penetrating 6 in. of wrought iron at 3000 yards' range, the 7.5-in. weapon now may perforate 6-3/4 in., and the 9.2-in. gun 9 in. of the hardest armour at corresponding range. The total weight of projectiles fired from the present-day cruiser in a minute is double, and the muzzle energy quadruple, the results attained by the cruisers designed at the close of the nineteenth century.[66] The modern cruisers steam at 23 knots, the power of the machinery in the _Argyll_ being 21,000 indicated horse-power, and in the _Defence_ 27,000 indicated horse-power. The machinery of the _Argyll_, which is typical, consists of four sets of triple-expansion engines, arranged in separate watertight compartments. The diameters of the cylinders are: high-pressure, 41-1/2 in.; intermediate-pressure, 65-1/2 in.; and the two low-pressure, each 73-1/2 in., all having a stroke of 42 in. At full power, developed with 138 revolutions, the piston speed is 966 ft. per minute. The cylinders are fitted with liners, and are steam-jacketed; forged steel is used for the liners of the high- and intermediate-pressure cylinders, and cast-iron for those of the low-pressure cylinders. The cylinder covers and pistons are of cast steel, the latter being of conical form. The high- and intermediate-pressure cylinders have piston valves, and the low-pressure cylinders flat valves. The cylinders are supported at the front by eight forged-steel columns, and at the rear by four cast-iron columns formed with guide-faces, and one forged steel column. The crankshaft is in four pieces, the high- and intermediate-pressure parts being interchangeable with each other, and the two low-pressure parts with one another. The shafts are hollow, and three-bladed propellers of manganese bronze are fitted to each. The condensers are entirely separate, and independent air pumps are fitted.