EVERY CREATURE ON THE EARTH lives under the warm, nurturing and protective blanket formed by the atmosphere. Yet all this air does more than trap the sun's heat and carry gases between plants and animals. It also presses down on our world with powerful force. At sea level, a single sheet of writing paper, when laid flat, sustains 6,111 newtons--about 1,400 pounds.

One might imagine that such a burden would stress living creatures enormously. But far from hurting organisms, the weight of the atmosphere proves absolutely essential for life. Liquid water could not exist on the earth were not atmospheric pressure sufficient to keep it from boiling rapidly away. And many vital biological processes, cellular respiration chief among them, fail if the air pressure falls too low.

Of course, atmospheric pressure decreases with altitude, and the earth's surface pokes up quite high in many places. Remarkably, humans can adapt to almost any elevation on the planet. Few other species can thrive both along the coast and between the peaks of the Himalayas, nearly six kilometers (3.7 miles) above sea level. By exercising some smarts, humans can keep warm and fed even in cold, harsh environments. But our adaptability may also be an evolutionary vestige: when our wandering ancestors crossed over mountain ranges, they had to adjust to the lower air pressure or die along the way. Can plants or other organisms that do not share our nomadic roots also adjust?

Amateurs can probe such mysteries of physiology thanks to Stephen P. Hansen, an innovative vacuum specialist in Amherst, N.H. Many science enthusiasts already know Hansen from the Bell Jar, his quarterly journal devoted to amateur experimentation with vacuum techniques. Inspired by the September 1965 installment of this column, which described a high-altitude chamber fashioned from a 30-gallon (113-liter) steel drum, Hansen developed a small and inexpensive version of this apparatus. Although technical complexities and ethical concerns would make it problematic to place one's pet hamster inside, this chamber is ideal for amateur investigations of less complicated organisms. Bacteria, insects or small plants, for example, serve as ideal test subjects.

Hansen's device consists of a stainless-steel mixing bowl and a Pyrex bowl. The Pyrex bowl makes a perfect see-through top for the chamber, and the stainless-steel bottom allows easy installation of vacuum ports and electronic sensors.

You form the airtight seal by pressing the bowls lip to lip into a gasket cut from a thin sheet of rubber. Hansen secured his 12-inch (30.5-centimeter) metal bowl from United States Plastics Corporation (800-537-9724 or 419-228-2242; catalogue number 84104). He bought a sheet of rubber from his local hardware store. You should be able to purchase a matching Pyrex bowl from just about any housewares merchant. Drill two holes in opposite sides of the steel bowl and epoxy 1/4-inch (five-millimeter) brass hose barbs into each. You can obtain such fittings from a well-stocked hardware or plumbing supplier.

Although just about any mechanical vacuum pump will effectively draw down the internal pressure, the high price of most models will also draw down your budget. But if you restrict your research to terrestrial conditions--pressures no less than those at the top of Mount Everest--you can get by with an inexpensive type. Hansen used a surplus dry-vane vacuum pump, which he procured from C&H Sales in Pasadena, Calif. (800-325-9465 or 626-796-2628; catalogue number PC9703), for $42.50. Regular vinyl tubing, affixed with steel hose clamps, makes fine vacuum line.

For experiments restricted to simulated altitudes no greater than 4,600 meters (about 15,000 feet), a pocket altimeter is the best choice to monitor the pressure. Edmund Scientific in Barrington, N.J. (800-728-6999 or 609-547-3488), sells one with a zero adjust for $34.95 (catalogue number 34,544). Just place the altimeter inside the chamber and read the equivalent elevation by looking at the scale through the Pyrex bowl. To mimic higher altitudes, you will need to hook up a vacuum pressure gauge; a Bourdon gauge would do. These units, which you can buy at an auto supply store, read in either millimeters or inches of mercury below ambient pressure. The illustration below shows the corresponding altitude.

Probably the most challenging problem you will encounter is regulating the pressure in your chamber. You could install an electronic sensor and control circuit that triggers the vacuum pump whenever the pressure climbs above a chosen set point. But such equipment is difficult to build, and commercial systems cost hundreds of dollars.

Fortunately, cheaper solutions exist. Hansen found that an $8 proportional relief valve works quite well. His choice is model number VR25, manufactured by Control Devices in St. Louis, Mo. (You can purchase one from W. W. Grainger: 773-586-0244; catalogue number 5Z-763.) The vacuum pump pulls air from the chamber, and the valve allows a weak flow of air to pass in from the outside. This adjustable leak prevents the full force of the vacuum pump from acting on the chamber, allowing the internal pressure to stay higher than it would otherwise. The needle valve allows a weak but constant stream of air to flush out any gases produced by living things inside.

Although it is a bit tricky to adjust the proportional relief valve to a particular pressure, once properly set, the chamber will maintain that level reliably. Still, you will have to keep your vacuum pump running all the while. Note that if the outside air pressure rises or falls, so will the pressure inside your chamber.

Certain biological experiments might require you to vary the temperature and light levels. Try putting the chamber on a windowsill, resting the stainless-steel bowl on a heating pad or submerging it in a bucket of ice water. For more precise adjustments, you may place the chamber under a full-spectrum light controlled by a timer and run a heat source from a thermostat. After struggling with homemade thermostats for years, I recently gave in and purchased a $165 unit from Omega Engineering in Stamford, Conn. (800-826-6342 or 203-359-1660; catalogue number CN8590).

If you go this route, you will also need to purchase 25 feet of type T thermocouple wire (Omega's catalogue number is PR-T-24SLE-25). It consists of two wires made from copper and constantan laid side by side. Cut a length of this double wire so that it is long enough to reach from the controller to the inside of your chamber. From one end, strip two or three centimeters (about an inch) from the tips of both wires and twist the exposed leads together. Once intertwined, a tiny voltage develops that is related to the temperature of the coupled wires. Attach the other ends of the wires to the controller.

You will have to feed the thermocouple wires (and any other electrical cables required for your particular experiment) into your chamber without creating leaks. Another hose barb serves well here. Pass the wires through the barb and then secure them in place by filling the tube with epoxy. After the epoxy sets, install this assembly through the stainless-steel bowl just as you mounted the others earlier.

The controller I used can deliver a maximum of five amperes at 120 volts AC--that is, it is limited to a power output of 600 watts. Omega sells many suitable heaters, but you could probably coat the sides of the steel bowl with a sprayable foam insulation and rest the exposed metal bottom on a small hot plate, such as the kind designed to keep your morning coffee warm in the cup.

Finally, you may find it useful to build two identical chambers and tie them together with a T-shaped joint. This duplication will allow you to run two experiments simultaneously, using one chamber for tests and the other as the control. At times, you may want to keep the second chamber at full atmospheric pressure. But should you wish to vary light level or temperature only, this arrangement will let you maintain the same low pressure in both chambers. Try growing tropical plants at alpine altitudes or test whether a fly tires more quickly in thin air. With a little imagination, you can continue such rarefied pursuits indefinitely.

Amateurs interested in vacuum-related experiments can subscribe to the Bell Jar. Send a check or money order for $20 to the Bell Jar, 35 Windsor Drive, Amherst, NH 03031. For more about this and other amateur science projects, visit the Forum section of the Society for Amateur Scientists's World Wide Web site. You can also write the society at 5600, Post Road, #114-341, East Greenwich, RI 02818 or call 1-877-527-0382.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.