Although all of them work on this same cycle, in which they resemble the engines of the motor-car, there are several much-used types of aero-engine in which the mechanical arrangement of the parts is quite different. Of these the best known is the famous Gnome engine which has a considerable number of cylinders arranged around a centre like the spokes of a wheel. The centre is in fact a case which covers the crank, and the cylinders are placed in relation to it just as the spokes are placed around the hub of a wheel. There is only one crank and all the connecting-rods drive on to it. Owing to their position around it they thus act in succession, giving a nice regular turning effort. Further, these engines differ from all others in that the crank is a fixture while the rest of the engine goes round, exactly the opposite of what we are accustomed to. The engine, in fact, constitutes its own flywheel. Rushing thus through the air, the cylinders tend to keep themselves cool, doing away with the need for cooling water and radiators. Consequently engines of this type are the very lightest known in proportion to their horse-power. A fifty horse-power engine can be easily carried by one man. It would be possible to go on much longer with this most interesting subject of engines, but having treated the three types which are most used in warfare, it is now time to pass on to something else. CHAPTER XV DESTROYERS Except for the submarine the most prominent craft during the war has undoubtedly been the destroyer. All warships are in one sense destroyers, since it is their prime duty to destroy other ships, so why should one particular kind of boat be given this name specially? Like many other of the terms which we use it is an abbreviation, a mere remnant of a fully descriptive title. "Torpedo Boat Destroyer" is what these ships are called in the Navy List. Even that full title, however, only tells us what their original purpose was: it leaves us very much in the dark as to the many various functions which they perform. The invention of the torpedo called for the construction of small boats whereby the new weapon could be used to best advantage, and so we got our torpedo boats. They in turn called forth another boat whose duty it was to run down and destroy them, and in that way we get our destroyers. From that bit of naval history we can almost see for ourselves what the characteristics of the destroyers must be. They have to be bigger than the torpedo boats, but as the latter were quite small the destroyers, though larger, are still comparatively small craft, latterly of about one thousand tons. Then they have to be very fast, in order to be able to chase the others and, finally, they need one or two guns, comparatively small so as not to overburden the ship and yet large enough to dispose of anything of their own size or smaller.