It could solve linear differential equations, but only linear equations. It had a long framework divided into sections with a couple dozen shafts buried through it. You could put different gears on the shafts using screwdrivers and hammers and it had "integrators," that gave [the] product of two shafts coming in on a third shaft coming out. By picking the right gear ratio you should get the right constants in the equation. We used published tables to pick the gear ratios to get whatever number you wanted. The limit on accuracy of this machine was the slippage of the mechanical wheels on the integrator.

That made me say, "Let's built electronic integrators and stick them into this machine instead of those wheel things." We added several dozen motors and amplifiers and circuits using over 400 vacuum tubes, which, as electronic things go, is not trivial. The radio has only five or six tubes, and television sets have up to 30. The Nova Chord organ was built prior to this and it has about 170 tubes. The Bush Analyzer was still essentially a mechanical device.

That led me to examine if I could find some way to multiply pulse numbers together so I didn't need gears -- then I could do the whole thing electrically. There's a theorem in calculus where you can use two integrators to do a multiplication. I talked with John Mauchley about it. Just who put in which part is hard to tell, but the idea of doing the integrations by counters was mine.

The ENIAC was the first electronic digital computer and could add those two 10-digit numbers in .00002 seconds -- that's 50,000 times faster than a human, 20,000 times faster than a calculator and 1,500 times faster than the Mark 1. For specialized scientific calculations it was even faster.

So it's a myth that ENIAC could only add, subtract, multiply and divide.