PART XVII. COMPOUND ENGINES. NAVAL COMPOUND ENGINES; COMPARATIVE MERITS OF THE SIMPLE AND THE COMPOUND ENGINE. COMPOUND ENGINES. The efforts of the engineering profession have long been directed to the reduction of the cost of the production of power, and for a lengthened period the question of the relative merits of the simple and compound engines occupied the attention of the engineering world. Many experiments have been made and much ability and ingenuity displayed, to prove that the compound system possessed no advantages over the ordinary simple engine working high-pressure steam; the most important of these experiments will be noticed presently. My annual report to the Department, as Chief of the Bureau of SteamEngineering, dated October 30, 1871, just after returning from a short tour of investigation of the subject, gives the facts at that time; and as they hold good to-day, I cannot do better than copy that part of the report which bears on the subject. It is as follows: The tour proved very instructive and interesting in many respects, especially in Great Britain, where immense fleets of iron ships are constructed and put afloat yearly, where ships are built for nearly all European nations, and where the British navy is Systematically improved and strengthened by additions of nearly twenty thousand tons of iron-clads annually, besides transports and unarmored vessels. The vast and varied experience of their constructing engineers and the sharp competition between rival building-firms have given rise to rapid improvements, and compelled the abandonment of many designs only a few years ago regarded as the best productions of engineering skill. The pressure of steam carried in marine boilers has gradually risen with corresponding increase in the extent to which expansion is carried, until boilers have been introduced, and are coming into universal use on board ship, in which the steam is kept from sixty to seventy-five pounds per square inch, and expanded in the cylinders from ten to fourteen times. The form of boiler used for generating the steam differs from the variety so long employed in all European steamers. It is built with a cylindrical shell of from 9 to 15 feet diameter, and of thickness from 4-inch to 14 inches, according to the pressure to be carried. It is sometimes from 9 to 10 feet long, and is placed in the vessel athwartships, with fire-rooms fore and aft in the ordinary way, and sometimes about 18 feet long, placed fore and aft and fired from either end. There are two or three cylindrical ines, according to diameter, in which are the grates, and above them a set of horizontal fire-tubes, through which the gases return to the front, thence to the funnel. This boiler is strong, of moderate cost, and generates steam freely and economically. The type of engine employed for working the steam is known as double-expansion or compound, the steam being admitted from the boilers first into a small high-pressare cylinder, there expanded to a third or fourth of its original pressure, then passed through a receiver into a large or low-pressure cylinder, where, after propelling the piston to the end of the stroke, it is exhausted into the condenser. This system of working steam expansively was introduced by Woolf seventy years ago; but at that date steam was used at a very low pressure, and the project met with no success. For many succeeding years the subject was discussed and experimented upon by engineers, but no advantages over the ordinary method of working steam expansively in a single cylinder were reached until a talented engineer of Scotland, Mr. John Elder, proprietor of the Fairfield Works, near Glasgow, several years ago took up the subject, and, in the face of immense opposition, zealously pursued it, designing and constructing every year several sets, each being more and more improved in design and detail, until they have been brought to the present state of perfection. Of the thirty-seven engineering works and iron-ship building yards on the Clyde, the Fairfield Works is now the most extensive and important. At the time of my visit this firm had already completed one hundred and thirty pairs of marine compound engines, and accompanying boilers, and had then twenty-two pairs under construction-all for ocean-steamers; besides, in their ship-yard, ten large iron vessels were on |