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Sleeping birds don’t fall, gas-pumps shut-off cleverly, hospitals cold shoulder germs

Q: How can birds sleep without falling off their perches? — Derek, Centerville, Indiana

The Aberrant Cardinal’s feet are locked on [Ed Wagner, US Fish & Wildlife Service]

A: Watch a sparrow or a finch or any of the perching birds land. She comes in with legs dangling, the toes find their purchase, the legs flex as she lands, and the toes lock on. Literally. Then she bobs around, checks for enemies, reaches for seeds, preens — whatever — perfectly secure, knowing that she’ll never fall. She even sleeps this way. When she leaves the branch, she jumps up, her legs straighten, and her toes unlock. Off she flies.

When a perching bird (passerine) lands and its legs flex, its flexor tendons automatically tighten. This causes the toes to close around a branch. The taut tendons lock the toes. The tendons stay tight until the legs straighten. Then the tendons relax, releasing the toes’ grip. That’s why birds can sleep without falling off — their toes fasten them to their perches.

The flexor tendons are narrow strips of gristle that extend from the leg muscles down the back of the tarsus bone and attach to the toes. The tarsus bone looks like part of the bird’s lower leg but actually corresponds to the five bones running from ankle to toes on a human’s foot.

Woodpeckers also hook their feet into the bark of a tree while they sleep, appearing frozen in a climb.

Further Surfing:

Humboldt State University: Avian features

National Audubon Society

Gas-pumps shut-off cleverly

Q: How does the gas station pump know to shut off when your tank is full? — Steve, Sturbridge, Massachusetts

A: A vacuum develops in the pump nozzle when the tank is full and that vacuum tells the pump to shut off.

Gasoline nozzle [Grenville Sutcliffe, Husky Corporation]

Next time you fill your tank, notice the nozzle. The nozzle "necks down" a short way past the handle and forms a narrow spout. Turn the nozzle over. There’s a small hole at the nozzle tip. Look closely. See how the "sensing" hole connects to a small vacuum tube that leads back up the nozzle. The tube connects the sensing hole at the nozzle tip with a diaphragm near the shut-off valve.

What you can’t see is a clever device (called a venturi) located also near the shut-off valve just before it necks down to spout, says Grenville Sutcliffe of Husky Corporation. It’s a ring with a channel through the ring. Gasoline must stream through the ring’s channel to reach the spout. The venturi ring also has four small conduits at right angles to the gasoline flow. These passages communicate flow pressure to the vacuum tube and a vacuum chamber above the diaphragm. The bottom of the diaphragm connects to the atmosphere.

Cut away diagram of a gasoline nozzle [Grenville Sutcliffe, Husky Corporation]

When you squeeze the handle trigger, gasoline pours through the hose into the narrow venturi’s channel, and picks up speed like a river running through a narrow gorge. As the gasoline stream moves faster through the venturi ring, its pressure drops and creates a vacuum. Like dominos: the pressure drops in the ring’s conduits, then in the vacuum tube, and, finally, at the tiny nozzle hole at the tip. Higher-pressure air outside the sensing port rushes in the small pipe and balances the pressure inside with out. The diaphragm stays in a neutral position. Gasoline continues to stream into the tank.

Eventually, the tank’s full. The gasoline in the fill pipe rises and covers the nozzle-sensing hole. Air can no longer rush into the nozzle hole to bleed off the venturi vacuum. The vacuum builds up in the vacuum chamber above the diaphragm. The higher atmospheric pressure on the underside of the diaphragm pushes the diaphragm up from the neutral position. This flips a switch in the automatic shut off. "Thunk!" The pump cuts off.

By the way, when you next time jam open the nozzle with the hold-open clip — think of Grenville Sutcliffe’s mother. Hazel Sutcliffe invented that nice convenience that enables you to stroll around while the tank’s filling.

Further Surfing:

Husky Corporation: When the nozzle won’t shut off

Arizona State Government: FAQ At the Pump

Hospitals cold shoulder germs

Q: Why are hospitals cold? — Kelly, Stammy, Connecticut

A: Hospitals are cold to discourage germs.

"The main reason hospitals are kept cold is that bacterial and germs tend to flourish more in warmer temperatures," says Lori Harris of the Desert Springs Hospital in Las Vegas, Nevada.

The temperature in the Desert Springs Hospital emergency and operating rooms is a brisk 68 degrees F (20 degrees C), whereas the lobby is a toastier 72 (22 degrees C).

(Answered Oct. 24, 2003)