The changes carried through in the two 4-cylinder engines were not major. The water-cooled area of the cylinder barrel was increased by nearly ten percent but the head remained only partially cooled. In hindsight, this consistent avoidance of complete cylinder-head cooling presents the one most inexplicable of the more important design decisions they made, as it does not appear logical. In the original engine, where the factors of time and simplicity were of paramount importance, this made sense, but now they were contemplating considerably increased power requirements, knowing the effect of temperature on both the cylinder and the weight of cylinder charge, and knowing that valve failure was one of their most troublesome service problems. Nor does it seem that they could have been avoiding complete cylinder cooling through fear of the slightly increased complexity or the difficulty of keeping the water connections and joints tight, for they had faced a much more severe problem in their first engine, where their basic design required that three joints be kept tight with only two sets of threads, and had rather easily mastered it; so there must have been some much more major but not easily discernible factor which governed, for they still continued to use the poorly cooled head, even carrying it over to their next engine series. Very probably they did not know the effect on detonation of a high-temperature fuel-charge. One of the new engines was intended for use in their future experimental flying and has become known as _No. 2._ It had a bore of 4-1/8 in., incorporated an oil pump, and at some time shortly after its construction a fuel pump was added. The fuel pump was undoubtedly intended to provide a metering system responsive to engine speed and possibly also to eliminate the small inherent variation in flow of the original gravity system.