with turnplates and sliding rails were twofold. First, they were relatively small (often no more than in length), which limited the wagon length that could be turned. Second, their switching capacity could only be accessed when the wagon was on top of them and still, which limited the total capacity of any wagonway. The railway switch, which overcame both of these problems, was patented by Charles Fox in 1832. As steam locomotives replaced horses as the preferred means of power, they became optimised to run in only one direction for operational ease and to provide some weather protection. The resulting need to turn heavy locomotives required an engineering upgrade to the existing turnplate technology. Like earlier turnplates, most new turntables consisted of a circular pit in which a steel bridge rotated. The bridge was typically supported and balanced by the central pivot, to reduce the total load on the pivot and to allow easy turning. This was most often achieved by a steel rail running around the floor of the pit that supported the ends of the bridge when a locomotive entered or exited. The turntables had a positive locking mechanism to prevent undesired rotation and to align the bridge rails with the exit track. Rotation of the bridge could be accomplished manually (either by brute force or with a windlass system), popularly called an "Armstrong" turntable, by an external power source, or by the braking system of the locomotive itself, though this required a locomotive to be on the table for it to be rotated. The turntable bridge (the part of the turntable that included the tracks and that swivelled to turn the equipment) could span from , depending on the railway's needs. Larger turntables were installed in maintenance facilities for longer locomotives, while short line and narrow gauge railways typically used smaller turntables. Turntables as small as in diameter have been installed in some industrial facilities where pieces of equipment are small enough to be pushed one at a time by humans or horsepower. Roundhouse In engine maintenance facilities, a turntable was usually surrounded, in part or in whole, by a roundhouse. It was more common for the roundhouse to only cover a portion of the land around a turntable but fully circular roundhouses exist, such as these preserved roundhouses: The roundhouse that serves as the basis for the Baltimore and Ohio Railroad Museum in Baltimore, MD The