The Moore School in 1934 and 1935 had built a differential analyzer; and, from that time on, the school had made a number of improvements in it. In 1941, with war imminent, the differential analyzer was put hard at work calculating tables for the Army’s Ballistic Research Laboratories. These tables were mostly firing tables, tables of the paths along which projectiles travel when fired—_trajectories_; obviously, you cannot fire a gun usefully, unless you know how to aim it. The amount of calculation of trajectories was so huge that Dr. Mauchly suggested that a machine using electronic tubes be constructed to calculate them. A good deal of discussion took place between men at the Moore School, men at the Ballistic Research Laboratories, and men from the Ordnance Department in Washington. A contract for research into an electronic trajectory computer was concluded with the Ordnance Department of the U. S. Army. Mauchly and one of the young electronics engineers studying at Moore School, J. Presper Eckert, Jr., set to work on the design. Gradually the design of a machine took form, and the crucial experiments on equipment were completed. In 1943, the design was settled as a special-purpose machine to calculate trajectories. Later on, the group modified the plans here and there to enable the machine to calculate a very wide class of problems. A group of Moore School electronics engineers and technicians during 1944 and 1945 built the machine, using as much as possible standard radio tubes and parts. Here, again, in spite of the successful progress of the electronic machine, the rumor that it was a “white elephant” was allowed to spread in order to protect the work from prying enemy ears. GENERAL ORGANIZATION The main part of Eniac consists of 42 _panels_, which are placed along the sides of a square U. Each of these panels is 9 feet high, 2 feet wide, and 1 foot thick. They are of sheet steel, painted black, with switches, lights, etc., mounted on them. At the tops of all the panels are air ducts for drawing off the hot air around the tubes. Large motors and fans above the machine suck the heated air away through the ducts. There are also 5 pieces of equipment which can be rolled from place to place and are called _portable_, but there is no choice as to where they can be plugged in. We shall call this equipment panels 43 to 47. Panels Now what are these panels, and what do they do? Each panel is an assembly of some equipment. The names of the panels are shown in the accompanying table. The arrangement of Eniac at the Ballistic Research Laboratories as shown in the table is slightly different from the arrangement of Eniac at Moore School. NAMES OF PANELS OF ENIAC PANEL NO. NAME (AND ADDITIONAL NAMES IN SOME CASES)