Depressions aren't what they used to be. The Great Depression of the 1930s ground on for at least three years, and arguably more. The depression that so many experts declared had begun on Oct. 19, 1987 held up for perhaps a couple of weeks. The Great Depression of 1989 lasted all of 90 minutes. Hardly time to organize a decent soup line.

Sooner or later we will wake up to the fact that computerized trading has turned stock markets into commodity pits, with the higher volatility that implies. The 10% slide that once took months to work through is now effected in a matter of minutes. For that matter, we might not even have noticed Friday's blowoff had not the ''circuit breakers'' been tripped at the Chicago Mercantile Exchange. These were the automatic trading halts introduced after the last crash that were supposed to calm the markets down in the event of a sharp movement in prices.

Instead, they succeeded only in forcing investors seeking to hedge their risk on the index futures markets into dumping their holdings of the underlying stocks on the New York exchange. The backlog of sell orders, moreover, spilled over into Monday morning, meaning the Depression lasted another 30 minutes longer than it might have.

At any rate, the Federal Reserve announced it would provide enough liquidity to forestall a credit crisis, and the panic passed. The only thing that would bring on a sustained market decline now, as one analyst said, is ''a return to double-digit inflation.''

PERFECTLY APPROPRIATE

Prepare for a sustained market decline. The chief effect of this episode will be to knock another chunk out of the Fed's anti-inflation credibility. This is not, mind, because of any mistake on the Fed's part. In a financial panic, there is often a sharp surge in the demand for money. It is perfectly appropriate for the central bank to meet this with a temporary increase in the supply of money, in order to keep the currency's value constant. That, after all, is its job.

It is not appropriate, and it is not the Fed's job, to prop up the stock market via a permanent easing in policy, any more than it should try to support a given level of growth in the economy at large. Yet that is how the Fed's policy choices are being presented, and how its actions are being interpreted. So great is the general concern over the levels of public and private debt that a recession has simply become impossible to contemplate. Even the threat of a recession, as implied by a market break, can no longer be tolerated.

It has now become the accepted wisdom in political and journalistic circles that central banks cannot achieve their stated goal of zero inflation. The costs of such a campaign, it is flatly asserted, would be too great. Calculations are run off laptop computers giving the precise amount of forgone output for each percentage point decline in inflation.

This is the Don Cherry school of macroeconomics. Among other things - his dog Blue, his taste in clothing, and his conviction that Europeans are ruining hockey - Cherry is known for his opposition to any move to enforce the rules more closely, on the grounds ''it would just slow the game down.''

This would be inarguable, if the players continued to trip, hook and slash each other with the same frequency as before. But it may just be that even hockey players are capable of learning that this sort of activity was no longer being tolerated, and would change their behavior accordingly.

So it is with monetary policy. Yes, if wage and price setters go on as before, then a tightening of demand will slow the economy and leave a good many out of work. But if they trim their demands to the new policy climate, then a monetary squeeze can lower inflation without affecting growth.

Economic agents will only adjust their pricing behavior - so averting a recession - if they think central banks would be willing to undergo a recession to get them to do so. The inability, or just the perceived inability, of monetary authorities to contemplate a recession is the best guarantee a recession will eventually occur, though not before another round of double-digit inflation.

We have now a kind of debtors' protection racket, with the stock market as the heavy. At the first sign monetary policy might not be sufficiently forthcoming to keep the debtors afloat, the market starts issuing threats. ''That's a nice little expansion you got here. Pity if anything should . . . happen to it.''

Even if the Fed does not buckle under this pressure today, each time the market swoons adds to the expectation that one day it will have to.