The _IBM Selective-Sequence Electronic Calculator_, constructed in the IBM laboratories, Endicott, N. Y., and installed in 1947 at the office of International Business Machines, 590 Madison Ave., New York, N. Y. Moore School of Electrical Engineering’s _EDVAC_ (Electronic Digital Variable Automatic Computer) being constructed partly at Moore School and partly elsewhere, and to be delivered to the Ballistic Research Laboratories, Aberdeen, Md. Harvard’s Sequence-Controlled Electronic Calculator _Mark III_, being constructed at the Harvard Computation Laboratory, and to be delivered to the Naval Proving Ground, Dahlgren, Va. We shall cover briefly (and perhaps a little technically) some of the main features of the first two of these machines; for, during 1948, they began to do problems. The other two had not been finished by the end of 1948 and so would be difficult to describe correctly, for mechanical brains _grow_, and design changes go on until they are finished—and even afterwards. Some information about these machines can be obtained from the organizations referred to above and from reports that should appear from time to time in some of the journals mentioned in Supplement 3. There is also a regular section entitled “Automatic Computing Machinery” in the quarterly _Mathematical Tables and Other Aids to Computation_, where it is likely that current information may be found. Harvard’s Mark II The Harvard Sequence-Controlled Calculator Mark II began to do problems under test during July 1947. This machine is at least twelve times as powerful as Mark I (see Chapter 6) and was constructed entirely by the Harvard Computation Laboratory. The machine contains about 13,000 relays of a new type that will operate reliably within ¹/₁₀₀ of a second. Numbers in the machine are regularly of 10 decimal digits between 1.000,000,000 and 9.999,999,999, inclusive, multiplied by a power of 10 between 1,000,000,000,000,000 and 0.000,000,000,000,001, inclusive. For storage of numbers, the machine has 100 relay registers totaling about 1200 decimal digits. Also, it can consult any one of 8 tape feeds for numbers and any one of 4 tape feeds for instructions. Effectively, the machine can read one number and one instruction from paper tape in ¹/₃₀ of a second. The machine performs all arithmetical and most logical operations. In every second it can carry out 4 multiplications, 8 additions (or subtractions), and 12 transfers. Division is performed by rapid approximation using the other operations.