Economists are uncovering peculiar similarities in events that range from the death of the steam-powered automobile in the early 1900s to the crash of the stock market last October. The decline of Sony's Betamax, they find, has something in common with the birth of the auto industry in Detroit. There are even similarities between the width of railroad tracks and the strange QWERTY arrangement of keys on a typewriter. In different ways, these rises and falls all defy economic doctrines of stability. All are examples of ``positive feedback'' _ things that get going in one direction and then keep going the same way. Like an avalanche, or a whistling microphone, or a buffalo stampede. Positive feedback helps explain what happened last Oct. 19, when corporate America lost nearly a quarter of its paper value in a single day. When prices dropped that day, buyers should have come in to pick up bargains. But trading strategies such as portfolio insurance locked the market into a suicidal loop of positive feedback, so each price decline triggered fresh sales. Portfolio insurers said they could protect investors against losses by selling their stocks and putting the money into interest-bearing Treasury bills. It worked _ until everybody else tried to sell at the same time. The loop was finally broken on Oct. 20, when the U.S. financial system was nearing gridlock. If all stocks had been covered by portfolio insurance, the Dow Jones industrial average theoretically could have sunk to zero. That is the power of positive feedback. In less frightening ways, positive feedback explains other things that defy traditional concepts of equilibrium. Once the VHS format got a slight edge over Sony's Betamax, for example, video rental shops began stocking more VHS tapes. That encouraged more people to buy VHS recorders, and so on. Now even Sony Corp. has added a VHS recorder line. Snowball effects likewise account for how railroad tracks that are 4 feet, 8{ inches wide crowded out other gauges, or why autos and electronics came to be concentrated in Detroit and Silicon Valley instead of evenly spread across the country. Herds of buffalo eventually get tired of stampeding. And at some point salaries for electronics engineers may get so high in Silicon Valley that it makes sense to locate elsewhere. Tired buffalo and overcrowding in Silicon Valley are examples of negative feedback, which leads to the cherished equilibrium of standard economics. In some cases, though, negative feedback never kicks in. Society becomes ``locked in,'' to use the phrase of W. Brian Arthur, a professor of population studies and economics at Stanford University who has done pioneering research in the field. Paul David, a colleague of Arthur at Stanford, has traced how the QWERTY typewriter layout has persisted in spite of the obvious superiority of the Dvorak Simplified Keyboard, which is 20 to 40 percent faster and can be learned in a few days. Then there is the clean, efficient steam-powered automobile, which exists no more. Years ago, smelly gasoline-powered cars gained some tiny advantage that grew into total dominance because the country was not big enough for two kinds of cars. In 1904, a man named William Fletcher with a cloudy crystal ball wrote: ``Unless the objectionable features of the petrol carriage can be removed, it is bound to be driven from the road, to give place to its less objectionable rival, the steam-driven vehicle of the day.'' Fletcher knew the power of positive feedback. He just bet on the wrong horseless carriage. In 1988, it's clear that many people don't know the power of positive feedback. For example, whiz kids in finance failed to foresee the damage that portfolio insurance could wreak in a falling stock market. Knowing about the snowball effect doesn't guarantee being able to do anything about it, but who knows? One thing could lead to another.