What fixed the exact cylinder size, that is, the "square" 4×4-in. form, is not recorded, nor is it obvious by supposition. Baker says it was for high displacement and low weight, but these qualities are also greatly affected by many other factors. The total displacement of just over 200 cu in. was on the generous side, given the horsepower they had determined was necessary, but here again the Wrights were undoubtedly making the conservative allowances afterwards proven habitual, to be justified later by greatly increased power requirements and corresponding outputs. The Mean Effective Pressure (MEP), based on their indicated goal of 8 hp, would be a very modest 36 psi at the speed of 870 rpm at which they first tested the engine, and only 31 psi at the reasonably conservative speed of 1000 rpm. The 4×4-in. dimension would provide a cylinder large enough so that the engine was not penalized in the matter of weight and yet small enough to essentially guarantee its successful operation, as cylinders of considerably larger bore were being utilized in automobiles. That their original choice was an excellent one is rather well supported by the fact that in all the different models and sizes of engines they eventually designed and built, they never found it necessary to go to cylinders very much larger than this.
[Illustration: _Figure 3._--First flight engine, 1903, installed in the Kitty Hawk airplane, as exhibited in the Science Museum. (Photo courtesy the Science Museum, London.)]
A second basic determination which was made either concurrently or even possibly in advance of that of the general form and size was in the matter of the type of cylinder cooling to adopt. Based on current practice that had proven practical, there were three possibilities, all of which were in use in automobiles: air, water, or a combination of the two. It is an interesting commentary that Fernand Forest's[10] proposed 32-cylinder aircraft engine of 1888 was to be air-cooled, that Santos-Dumont utilized an air-cooled Clement engine in his dirigible flights of 1903, and that the Wrights had chosen air cooling for their shop engine. With the promise of simplicity and elimination of the radiator, water and piping, it would seem, offhand, that this would be the Wrights' choice for their airplane; but they were probably governed by the fact that not only was the water-cooled type predominant in automobile practice, but that the units giving the best and highest performance in general service were all water cooled. In their subsequent practice they never departed from this original decision, although Wilbur Wright's notebook of 1904-1907 contains an undated weight estimate by detailed parts for an 8-cylinder air-cooled engine. Unfortunately, the proposed power output is not recorded, so their conception of the relative weight of the air-cooled form is not disclosed.