Jack Janes, age 9, Jeffersonville, Ohio,
How does a barometer work.?
Lets play a game at weighing everything in the classroom everything. We put desks' seats, books, papers, pencils and all the gadgets onto our, imaginary scales. Then we weigh Andy’s favorite teacher, Miss Patience and all the students and total up the figures. Andy says your total is short by about 1,000 pounds, half a ton. What did you forget? The air, of course. Are you surprised to learn that air weighs so much?
The atmosphere stretches hundreds of miles above our heads. Imagine the weight of air pressing down on the world. Air is flimsy stuff, forever thinning out here, piling up there and blowing hither and thither, Hut air pressure, the weight of the atmosphere above any spot, can be measured This is the job of the barometer. Normal air pressure at sea level is 141 pounds over every square inch. Higher up, there is less air pressure coming down from above. Three miles above the ground, normal air pressure is 74 pounds over every square inch.
A barometer acts as a weather prophet when it registers the changing air pressure around us, An altimeter is a barometer that tells a pilot how high he is flying from the weight of the air above him. The weather barometer is usually made of glass and mercury. The altimeter is usually worked with a vacuum box and a spring.
Maybe your Papa taps his weather barometer on his way through the front door. It has a needle that swings around and points to rain, fair or change. Papa knows whether to take his galoshes or leave them home, You can make a simple model of this handsome instrument for yourself. It gives you a chance to play with that wonderful, shiny, runny stuff called mercury. You will need a glass tube three feet long and sealed at one end. You will need a smallish ;lass dish and, of course, lots of fascinating mercury.
Pour mercury an inch deep into the dish, Fill the tube with mercury to the brim. Seal the tube with your thumb and turn it upside down. Gently put the top of the tube under the level of the mercury the dish without spill it. Now you can take away your thumb. Hold the tube straight up and steady and watch.
The mercury falls down the tube to a certain level then stops. Why? The atmosphere is pressing down on the mercury in the dish. Its weight is exactly balanced by the column of mercury in the tube. Tomorrow the pressure may be less and the mercury in the tube will fall. Or it may increase and push the column of mercury up higher.
Papas handsome wall barometer is finished with a scale of degrees and mounted on a fine wooden panel. The pool of mercury is almost sealed in a glass tube. On the scale, the air pressure is normal when the column of mercury is 29.92 inches high. Lowered air pressure usually means that good weather is on the way, The column of mercury rises. Higher, or heavier air pressure often means that stormy weather is ahead, The mercury falls A needle is attached t o the barometer to translate the scale on the barometer into words. This needle swings around to give Pop his morning weather forecast.