[Illustration: Propeller estimates, made by the Wrights 8 months before the flights of December 1903. Their formulas resulted in the highly efficient propellers which were used in the first Wright Flyer. These were 8½ feet from one canvas-covered tip to the other. Top view shows the front, bottom view, the rear.] Owing to certain peculiarities of design, after several minutes’ run the engine speed dropped to less than 75 percent of what it was on cranking the motor. The highest engine speed measured developed 15.76 horsepower at 1,200 revolutions per minute in the first 15 seconds after starting the cold motor. After several minutes’ run the number of revolutions dropped rapidly to 1,090 per minute, developing 11.81 brake horsepower. Even so, the Wrights were pleasantly surprised since they had not counted on more than 8 horsepower capable of driving a machine weighing only about 625 pounds. Having a motor with a power output of about 12 horsepower instead of 8, the Wrights could build the machine to have a larger total weight than 625 pounds. The motor was started with the aid of a dry-battery coil box. After starting, ignition was provided by a low-tension magneto, friction-driven by the flywheel. No pump was used in the cooling system. The vertical sheet-steel radiator was attached to the central forward upright of the machine. When the brothers began to consider designing propellers, they unhappily discovered that the forces in action on aerial propellers had never been correctly resolved or defined. Since they did not have sufficient time or funds to develop an efficient propeller by the more costly trial-and-error means, it was necessary for them to study the screw propeller from a theoretical standpoint. By studying the problem, they hoped to develop a theory from which to design the propellers for the powered machine. The problem was not easy, as the Wrights wrote: What at first seemed a simple problem became more complex the longer we studied it. With the machine moving forward, the air flying backward, the propellers turning sidewise, and nothing standing still, it seemed impossible to find a starting point from which to trace the various simultaneous reactions. Contemplation of it was confusing. After long arguments we often found ourselves in the ludicrous position of each having been converted to the other’s side, with no more agreement than when the discussion began.