High-speed automatic computing machines are now established as necessary tools for research and development. Such machines usually work with a binary number system, the numbers being represented by trains of electric pulses. An automatic machine must have a “memory” or storage system capable of storing a very large number of pulse patterns. The two main types of storage now in use are cathode-ray-tube systems and delay-line systems. A delay-line system utilizing quartz-crystal-generated ultrasonic waves propagated in a mercury column, was used in the Ace Pilot model because of its intrinsically higher speed and its reliability. It has an apparent disadvantage with respect to cathode-ray-tube systems in that any given item of stored information is available only once per circulation period, but it is shown that this can be obviated by a suitable logical design and the so-called “optimum” system of programming. The Ace Pilot Model will have a 3-tier storage system, i.e. long delay-lines (32 numbers), short delay-lines (up to 2 numbers) and a magnetic drum. It is shown that with this system and optimum programming the overall speed of computation can be practically as high as if all items of stored information were immediately available.The delay lines and the associated circuit technique are described in the paper.