six cylinders to their engines, each triplet consisting of a high-pressure between two low-pressure ones. The initial pressure in the boilers was 60 lbs.; steam was cut off in the middle cylinder of each group at about half-stroke, and exhausted thence into the other two. This deserves notice as being the first instance of the use of the compound engine in the navy, though it was not long after the trials of the 'Constance' that Messrs Humphrys & Tennant employed a simpler form of the same principle in the 'Pallas.' The three ships were sent for a cruise in company to Madeira and back, but no very grand results were obtained. The 'Octavia' became commodore’s ship during the Abyssinian War, the 'Constance' went to the West Indies, and the 'Arethusa' to the Mediterranean. After one commission they were never employed again; they were admitted to be failures, and their hulls have long since gone to the shipbreaker, their engines to the scrap heap.
Let us look, then, at the state of steam navigation in the Royal Navy in 1865, the year that marks the close of the first half of the fifty years that this book deals with, and great as the progress made, and radical as the changes introduced in this quarter of a century may seem, they certainly did not surpass in importance either the progress or the changes of the next. In 1865 the paddle-wheel as a mode of propulsion for fighting ships had become entirely obsolete, its place having been taken by the screw. The pressure of the steam in the boilers had increased from 7 lbs. in the
