The men march through tropical heat that threatens to steam them like so many soft-shelled clams. Not a hundred yards away, in another climate-controlled room , a group battles an arctic cold that makes a New England winter seem balmy. In a building nearby, torrential rainstorms pelt tents and other gear, hunting for a weak seam or flaw that could give way.
The United States military knows that troops on the battlefield need more than the latest in sophisticated weaponry. They need food, clothing, and shelter that won't let them down.
That's where the US Army's Natick Research and Development Laboratories come in. Since 1954 scientists at Natick's test chambers and laboratories, situated on a 78-acre plot in this Boston suburb, have developed a large number of products that improve not only the comfort but the survival rate of men and women in uniform. In addition, many of the laboratories' successes have been snatched up by private industry. It's estimated that military research or testing has played a part in 20 percent of the products found on a typical grocery store's shelves. Many of those innovations have come from the laboratories here.
Among the Natick products used by consumers:
* Goretex, the fabric that keeps water out while letting the body ''breathe.'' Natick labs, working with the former Du Pont chemist who invented it, found the material could be used in clothing, sleeping bags, and tents.
* The retort pouch, a light, flexible substitute for cans that has been called the first major advance in food packaging since the can itself. Already in wide use in Western Europe and Japan, the pouch is expected to be a popular container on US shelves in the near future.
* Freeze-dried foods, a favorite of backpackers and hikers, were developed for US soldiers in Vietnam. More recently, compression techniques allow a hockey-puck-size block of a food such as string beans to expand to the equivalent of 15 cans by adding water.
* So-called bulletproof vests used by police officers and VIPs. Designed of layers of Kevlar, a Du Pont product, the thin, lightweight material was originally developed to protect infantrymen from shell fragments.
* Flaked and formed meat that looks and tastes like expensive cuts but uses much more of the animal. Natick researchers are now trying find ways to make edible ''trash fish'' more palatable by using packaging that makes the taste and texture comparable to a well-known species.
In all, taxpayers are spending $65 million a year at Natick to keep about 1, 000 civilians and 100 military personnel busy designing everything from a new helmet for foot soldiers (it's lighter and won't wobble around) to easy-to-slip-on dog booties (for sled teams in arctic regions).
''There's no other research institute like this in the world,'' says Harvey W. Keene, a civilian spokesman for the laboratory. He estimates that the facility works on ''maybe a couple of hundred'' projects at any one time, although not all are major innovations.
The Natick facility doesn't design weaponry and doesn't manufacture products. Rather, it creates new products and then carefully writes the specifications that manufacturers who supply the military must follow. The laboratories are prohibited by law to do research on products available commercially. ''That's why we have to keep abreast of what private industry is doing,'' says Mr. Keene. Firemen's boots, work gloves, and bath towels are among off-the-shelf products used by the military.
But when no satisfactory product exists, the Natick scientists go to work. Some of them have become experts in anthropometry, the science of measuring the human body and its variations. Using their skills, they designed new fatigue uniforms for the Army, which resist detection by infrared sensors of enemy troops. (Less exotic togs, such as the new uniforms for the US Postal Service, are a Natick product too.) Among the innovations now under testing is a ''liquid-cooled garment'' for tank crews and others who must work in hot environments where air conditioning is impractical. Professional race car drivers are reportedly interested in it.
Some of the testing has a bit of whimsy - like the machine that takes boots on simulated 50,000-step hikes. But the work is deadly serious, too. One project is tackling the problem of feeding troops who are under attack by chemical or biological weapons.