An electronic serial binary digital computer is described, using mercury acoustic tubes for the main store of 1 msec access time. A low-speed store of magnetic drum type raises the total digit capacity to 40 960 binary digits, or 2 048 separate numbers. The command code is of the "two-address" type in which a source and a destination are specified. Programmes are placed in the high-speed store and commands are adopted sequentially at a rate of 500 per sec. Full facility for making conditional or absolute changes in the uniform sequence of commands is provided.