“‹A›” represents the total area of the supporting surface; “‹a›” represents the total area of the tail; ‹HP› represents the horse-power by Prony brake measurement. “Horse-power by formula” is given by Maxim’s formula:[15] rev.×diam. of propeller×pitch×thrust ‹HP› = ------------------------------------. 33,000 (This formula was not in use at the time of the rubber-motor experiments, for which the thrust was not taken. It appears to assume the conditions where the screws from a fixed position move a mass of still air, are the same as those of free flight. Its results, however, are in better agreement with experiment than might be anticipated.) “Flying-weight” means everything borne in actual flight, including fuel and water. [p016] Remembering that the principal object of all these experiments is to be able to predict that setting of the wings and tail with reference to the center of gravity which will secure horizontal flight, we must understand that in the following tables (see No. 30) the figures ‹CP›_m = 1516.5 cm. mean a prediction that the center of pressure of the sustaining surfaces in motion (‹CP›_m) is to be found in a certain position 1516.5; that is, 16.5 cm. in advance of the line joining the propeller shafts. This prediction has been made by means of previous calculation joined with previous experimental adjustment. We know in a rough way where the ‹CP› will fall on the wings when they are exposed independently if flat, and at a certain angle, and where it will fall on the tail. From these, we can find where the resulting ‹CP› of the whole sustaining surface will be. It would seem that when we have obtained the center of gravity by a simple experiment, we have only to slide the wings or tail forward and back until the (calculated) center of pressure falls over this observed center of gravity. But in the very act of so adjusting the wings and tail, the center of gravity is itself altered, and the operation has to be several times repeated in order to get the two values (the center of pressure and center of gravity) as near each other as they are found in the above-mentioned table, our object being to predict the position which will make the actual flight itself horizontal. How far this result has been obtained, experiment in actual flight alone can show, and from a comparison of the prediction with the results of observation, we endeavor to improve the formula.