(FORTUNE Magazine) – Here's an economic puzzle: While executives are sitting around brooding over the terrible state of business, many have been investing vigorously in | equipment and machinery to increase efficiency and capacity or to replace worn-out gear. Real purchases shot up at a 17% annual rate in the third quarter from the second. This uncharacteristic strength is not sustainable -- the surge was part mirage, and the economy remains mired in a painfully slow recovery. Employment is stagnant, and consumers are no less gloomy than businessmen. Nonetheless, equipment spending is rising, in contrast to spending in like periods of all other business cycles since 1953, and recent orders are encouraging (see graph). We expect equipment investment in 1992 to post about a 5% gain. Computers account for much of that third-quarter upswing -- and for much of the mirage. Spending on them is up 12% so far this year, adjusted for inflation. Further price cuts, coupled with the ever-growing technological capabilities of new machines, will continue to spur investment next year. Sales of smaller units, from workstations to palmtops, should be especially brisk. InfoCorp of Santa Clara, California, a market research firm, predicts 17% unit growth for these models. For the whole industry, including software, Gartner Group of Stamford, Connecticut, expects 9% growth in revenue.

But the industry's contribution to reported economic growth will shrink as of December 4, when the government, in one of its periodic revisions, rebases calculation of real GNP to 1987 dollars from 1982 dollars. The 1982 figures overstate the current computer share of overall equipment investment: Computer prices have been falling, and prices of other equipment have been rising. Another overstatement comes from auto purchases by business, which roared ahead in the past couple of quarters at unsafe speed. While automakers have cut back on schemes to hype fleet sales, the latest investment numbers reflect aggressive leasing to consumers, another subsidized merchandising program to move the metal. The government records these as business purchases because the owner of the vehicle is a leasing company. If the deals end, or even if they are not promoted further, the pace of ''business'' auto sales will slacken. SPENDING ON commercial aircraft continues to advance -- it's up some 20% so far in 1991. No boom can last forever, especially when the customers have been buying heavily amid a profit squeeze. Orders have trended down for the past two years, and some airlines have canceled deliveries. But backlogs remain huge, and purchases will keep growing. Boeing says output won't be affected + until the fourth quarter of 1992. The most compelling argument for decent equipment growth is the continued drive for productivity improvement. Business has clearly used the recession to target unit labor costs, which means not only laying people off but also lifting the output per hour of those remaining with efficient new machine tools and the like. Indeed, third-quarter productivity was higher than a year earlier, no mean feat at a time when production was falling. Manufacturers, in particular, will want to keep the share of international markets they won in the past decade, partly through productivity increases. The National Association of Manufacturers, for example, is hammering the export theme to its membership. The big downer in business spending is construction, which will decline about 12% this year and 5% next. Offices, hotels, motels, and shopping centers, in monumental oversupply, will be down about 20% both years. As textbook excesses that must be corrected over a long time, these markets will stay disastrous no matter how low interest rates go or how the economy performs.

BOX: OVERVIEW

-- Investment will grow about 5% next year. -- Sales of smaller computers should be especially brisk. -- Aircraft will climb, despite recent order cutbacks.

CHART: NOT AVAILABLE CREDIT: FORTUNE CHART CAPTION: CAPITAL EQUIPMENT BENT ON PRODUCTIVITY Manufacturers will spend to get more efficient. This new Giddings & Lewis machining center combines several production steps in a single location.