An engine which was not developed to the same high point in this country as the Liberty motor is the rotary engine, of which the Gnome Monosoupape or Clerget are perhaps the best-known types. These were favorites with airmen flying fighting scout-planes. They weighed practically nothing, for an engine. A one-hundred-horse-power motor weighed only two hundred and sixty pounds, and it was a splendid type for fast work. Briefly, the power generated by the explosions in the cylinders, operating against two centers of pressure, gave a rotary motion to the cylinders and crankcase, revolving around a stationary, hollow crankshaft. Cylinders and crankcase were bolted together, and the cylinders looked like the blades of an electric fan. There was always an odd number of cylinders, so that there would be no dead-centers, no point at which two opposing strains would be balanced, causing the engine to stop. The propeller was bolted on a nose cap which revolved with the engine. This type of engine is not likely to be used to any extent for commercial flying, or even flying for sport. It is expensive, very wasteful of gasolene and oil, and difficult to keep in repair. For men who may have had some experience in the assembly of airplanes at factories, or of rigging them at flying-fields, there is great opportunity. Expert riggers who know their craft are few and hard to get. They are invaluable for maintaining a machine in flying condition. The use of airplanes in this country will require men for rigging, for truing up the wires and struts. Each airplane must be overhauled after a few hours of flight to discover hidden weaknesses and to tighten sagging wires. Rigging an airplane has some resemblance to rigging a ship for sailing. The first requisite is to see that the machine is properly balanced in flying position. There is a number of minute measurements which come with the blue-print of every machine and which must be followed out to the letter to get the most successful results. An important detail is the pitch of the planes, or the angle of incidence, as it is called. This is the angle which a plane makes with the air in the direction of its motion. Too great a pitch will slow up the machine by offering too great a resistance to the air; too small an angle will not generate enough lift. The tail plane must be attached with special care for its position. Its angle of incidence must exactly balance the plane, and it must be bolted on so that there is no chance of it cracking off under strain.