he remarked that for the purposes required "the memory needs to be very large indeed by comparison with standards which prevail in most valve and relay work, and [so] it is necessary to look for some more economical form of storage", and that memory "appears to be the main limitation in the design of a calculator, i.e. if the storage problem can be solved all the rest is comparatively straightforward". The ACE implemented subroutine calls, whereas the EDVAC did not, and what also set the ACE apart from the EDVAC was the use of Abbreviated Computer Instructions, an early form of programming language. Initially, it was planned that Tommy Flowers, the engineer at the Post Office Research Station at Dollis Hill in north London, who had been responsible for building the Colossus computers should build the ACE, but because of the secrecy around his wartime achievements and the pressure of post-war work, this was not possible. Pilot ACE Turing's colleagues at the NPL, not knowing about Colossus, thought that the engineering work to build a complete ACE was too ambitious, so the first version of the ACE that was built was the Pilot Model ACE, a smaller version of Turing's original design. Turing's assistant, Jim Wilkinson, worked on the logical design of the ACE and after Turing left for Cambridge in 1947, Wilkinson was appointed to lead the ACE group. The Pilot ACE had fewer than 1000 thermionic valves (vacuum tubes) compared to about 18,000 in the ENIAC. It used mercury delay lines for its main memory. Each of the 12 delay lines was 5 feet (1.5 m) long and propagated 32 instructions or data words of 32 bits each. This ran its first program on 10 May 1950, at which time it was the fastest computer in the world; each of its delay lines had a throughput of 1 Mbit/s. The first production versions of the Pilot ACE, the English Electric DEUCE, of which 31 were sold, were delivered in 1955. MOSAIC A second implementation of the ACE design was the MOSAIC (Ministry of Supply Automatic Integrator and Computer). This was built by Allen Coombs and William Chandler of Dollis Hill who had worked with Tommy Flowers on building the ten Colossus computers. It was installed at the Radar Research and Development Establishment (RRDE) at Malvern, which later merged with the Telecommunications Research Establishment (TRE) to become the Royal