The compression-release mechanism was kept relatively simple, considering what it was required to accomplish. A small non-revolving shaft was located directly under the rocker arm rollers that actuated the exhaust valves. Four slidable stops were placed on this shaft, each in the proper location, so that at one extreme of their travel they would be directly underneath the rocker roller and at the other extreme completely in the clear. They were positioned along the shaft by a spring forcing them in one direction against a shoulder integral with the shaft, and the shaft was slidable in its bearings, its position being determined by a manually controlled lever. When the lever was moved in one direction the spring pressure then imposed on the stops would cause each of them to move under the corresponding rocker roller as the exhaust valve opened, thus holding the exhaust valve in the open position. When the shaft was moved in the other direction the collar on the shaft would mechanically move the stop from underneath the roller, allowing the valve to return to normal operation. [Illustration: _Figure 8._--Development engine No. 3, 1904-1906, showing auxiliary exhaust port, separate one-piece water-jacket block. (Photo by author.)] If the 1903 engine is the most significant of all that the Wrights built and flew, then certainly the _No. 2_ unit was the most useful, for it was their sole power source during all their flying of 1904 and 1905 and, as they affirmed, it was during this period that they perfected the art, progressing from a short straightaway flight of 59 seconds to a flight controllable in all directions with the duration limited only by the fuel supply. It is to be greatly regretted that no complete log or record was kept of this engine. The Wrights again exhibited their engineering mastery of a novel basic situation when, starting out to make flight a practical thing, they provided engine _No. 3_ to be used for experimental purposes. In so doing they initiated a system which continues to be fundamental in the art of providing serviceable aircraft engines to this day--one that is expensive and time consuming, but for which no substitute has yet been found. Their two objectives were: improvement in performance and improvement in reliability, and the engine was operated rather continuously from early 1904 until well into 1906. Unfortunately, again, no complete record exists of the many changes made and the ideas tested, although occasional notes are scattered through the diaries and notebooks.