Babbage’s vision, in essence, was digital computing. Like today’s devices, such machines manipulate numbers (or digits) according to a set of instructions and produce a precise numerical result.
Yet after Babbage’s failure, computation entered what English mathematician L. J. Comrie called the Dark Age of digital computing—a period that lasted into World War II. During this time, machine computation was done primarily with so-called analog computers. These devices model a system using a mechanical analog. Suppose, for example, one wanted to predict the time of a solar eclipse. To do this digitally, one would numerically solve Kepler’s laws of motion.
Before digital computers, the only practical way to do this was hand computation by human computers. (From the 1890s to the 1940s the Harvard Observatory employed just such a group of all-female computers.) One could also create an analog computer, a model solar system made of gears and shafts that would “run” time into the future. Before World War II, the most important analog computing instrument was the Differential Analyzer, developed by Vannevar Bush at the Massachusetts Institute of Technology in 1929. At that time, the U.S. was investing heavily in rural electrification, and Bush was investigating electrical transmission. Such problems could be encoded in ordinary differential equations, but these were very time-consuming to solve. The Differential Analyzer allowed for an approximate solution without any numerical processing. The machine was physically quite large—it filled a laboratory—and was something of a Rube Goldberg construction of gears and rotating shafts. To “program” the machine, researchers connected the various components of the device using screwdrivers, spanners and lead hammers. Though laborious to set up, once done the apparatus could solve in minutes equations that would take several days by hand. A dozen copies of the machine were built in the U.S. and England.
One of these copies belonged to the U.S. Army’s Aberdeen Proving Ground in Maryland, the facility responsible for readying field weapons for deployment. To aim artillery at a target of known range, soldiers had to set the vertical and horizontal angles (the elevation and azimuth) of the barrel so that the fi red shell would follow the desired parabolic trajectory—soaring skyward before dropping onto the target. They selected the angles out of a firing table that contained numerous entries for various target distances and operational conditions.
Every entry in the firing table required the integration of an ordinary differential equation. A human computer would take two to three days to do each calculation by hand. The Differential Analyzer, in contrast, would need only about 20 minutes.
Source of Information : Scientific American September 2009