Unlike a digital computer, which performs calculations strictly upon numbers or symbols, an analog computer translates continuously varying quantities such as temperature, pressure, weight, or speed into corresponding voltages or gear movements. It then performs "calculations" by comparing, adding or subtracting voltages or gear motions in various ways, finally directing the result to an output device such as a cathode-ray tube or pen plotter on a roll of paper. Common devices like thermostats and bathroom scales are actually simple analog computers: they "compute" one thing by measuring another; they do not count. The earliest known analog computer is an astrolabe. Built in Greece during the first century b.c., the device used pointers and scales on its face and a complex arrangement of bronze gears to predict the motions of the sun, planets, and stars. Other early measuring devices were also analog computers. Sundials traced a...