The Atmosphere In Motion
( Originally Published 1891 )
'" UGH ! how cold the air feels this morning ! " Dick's pearly teeth chattered as he spoke, and the gray hair bristled all along his back.
Poor little Chip looked like a ball of trembling fur, as he squeaked, " I wish we might have only warm south winds in our valley."
" Oho, my pretty grumbler ! " said Bunny.
Was it not the north wind that chilled the vapor-wings last April, and sent our early showers from the clouds as they came flying northward ? But listen ! what is the brooklet singing this morning ? "
"'Whichever way the wind cloth blow,
"'My little bark sails not alone,
" 'So whichever way the wind doth blow,
-CAROLINE A. MASON.
So soft and sweet the song, it seemed as if the brook o'erflowed with liquid music ; and that each dimpled wave, rippling upon the shore, poured its rich melody over the listening pebbles, till they burst forth in purling echoes.
The gentle spell was broken by the robin's cheery voice, calling, " Come, let us go down and ask the waves to tell us why the cold winds blow, for I fear that winter is not far off."
"It will be a long story," bubbled the brook; " but you can all help me tell it.
" Near by where Bunny sits, there is a very queer plant. Its, leaves look as if they were sewed together to form deep, -narrow pitchers."
" 0, you mean the pitcher-plant among the cranberry vines ! Here it is full of water ! " shouted nimble Chip, who was first to peep in.
" Bunny, I wish you would nip off one of the coarse grass-stems, so that the top of the stubble will be just on a level with the mouth of the pitcher," said the brooklet.
" That is well done.
" Now, Chip, you may find a piece of dry twig as long and wide as the pitcher, — one that will just fill it. Dick may bring a stone of the same size Bunny may get me several small pebbles ; Red-breast may gather some gray moss from the north side of the old pine-tree, and we shall be ready."
Away they hurried, and soon came back with their light loads, wondering what stones, sticks and moss had to do with cold north winds.
" Can you tell me, Chip," babbled the brook, "which is heavier, the piece of wood you have brought, or the water in the pitcher ? "
That was a poser for the bright chipmonk, and set him thinking.
" And which is lighter, Dick, your stone or the water ? "
Then Dick also put on his thinking-cap.
"Now it is your turn, Bunny. You brought the pebbles and — "
" I have it ! " broke in Dick. " I can find out which is heavier," and he dropped his stone into the green pitcher, spilling the water over its sides.
" The stone is heavier than water, for it bends the hollow leaf far below the top of the grass-stubble. Here, Chip, put in your wood and weigh. it."
The chipmonk placed the twig in the hollow leaf and, to his surprise, it rose above the stubble. "Who would have thought that pine is so much lighter than water ? " chirped the merry fellow.
" In with the pebbles, Bunny ! " bubbled the brook, and down went the pitcher below its water-mark on the grass-stem. Then came the dry moss, and the pretty . folded leaf stood nearly upright.
" Now bring them all here," babbled the silver stream. " Throw them into the water, and see which will float."
Down went the rock and pebbles, but the wood and moss sailed off like tiny boats. Then they weighed dry bark, leaves, feathers, grass, old nails and bits of glass, and threw them into the brook-let. At length the rabbit stopped. His eyes sparkled, and his pretty face was all aglow.
" What is it, Bunny ?" asked Dick.
" We need not weigh any more in the pitcher-plant. Don't you see that only those things that are lighter than water will float ? Wood, moss, bark, leaves, grass and feathers do not sink be-cause they are not so heavy as water."
" Then tell me, bright rabbit, which is lighter, water or air ? " came the brooklet's soft voice.
"Air, of course," chattered Bunny. " It floats on water. Besides, it doesn't bend our pitcher so low as the grass-stubble."
" Vapor must be lighter than air," added Dick, " for the clouds float far above our heads. And I know that very warm breath rises in winter time, for I have often seen it float away like water-dust."
" One day," piped the robin, " the dry grass in our meadow was afire. When I tried to fly over it, the hot air came rushing up, and almost stifled me. It lifted large burning leaves and thick smoke higher than the tree-tops.
" I could fly round and round the meadow, near the ground, and could feel cold air rushing towards the fire. But every time I flew above the flames, my feathers were singed."
" You have used your bright eyes well, pretty Redbreast," rippled the stream, "and what you have said is true. Warm air will float on cool air, just as a stick will float on water, and for the same reason.
"There are often many gases besides air and vapor sailing above and around us. Altogether, they are called the ' atmosphere.' When heated in one spot, the cooler atmosphere round about will flow under and float the warmer. We feel it in motion, and then call it ` wind.'
" Thus you see that vapor and other gases are moved, not by the air, but with it, — the same force moving all by drawing the heavier under the-lighter.
" It is the same force that sinks the rock, and draws the water under the stick. It causes the heavy cold vapor and air to float the light warm gases of the same kind. It makes the raindrops fall, and the brooklet flow down hill. It is the wonderful force called ` gravity,' without which everything on the earth's surface would fly off into space."
"But what heats the atmosphere, and so makes it possible for gravity to move it about ? " queried Dick.
" Can't you tell me what it is ? " asked the brooklet.
It may be the sun," was the squirrel's answer. "But why are some parts of our valley warmer than others ? The sun shines on all alike, yet the sand-pit is much warmer than the meadow.
" Then there is our great flat rock which is so hot when the sun shines, while the old log beside it is only warm. At night the sand and rock are much cooler than the grass-land and wood.
" The top of the hill, too, is often cooler than its foot ; yet the sun shines on both. And the sand-pit is warmer than the top of the nut-tree, although the boughs are nearer the sun."
" I think 'that the heat must come from the earth," sang the robin, " for the higher I fly, the colder I find the air."
" How can that be ? " asked Bunny. " My burrow is cool all day long."
" But if the sun heats the atmosphere, why is it not warmer among the clouds than down here ? It is nearer the sun up there," chirped Redbreast.
" I cannot answer that," replied honest Bunny. " But if the heat comes from the earth, why is the air cooler when a cloud hides the sun ? "
0, I know ! whispered the wild-flower.
" When the sun shines very brightly, our blossoms often send out a cloud of perfume to keep the hot rays from wilting them. Perhaps the great clouds above our heads also stop a part of the sun's heat.
" Wait a minute ! " cried Chip ; and up he ran into a tree that hung over the sand-pit.
Soon he called from his lofty perch, " It is much cooler here than down near the sand. Now I can tell you how the atmosphere is heated. First, the sun heats the surface of the earth, and then the surface warms the atmosphere."
" I believe that Chip is right," said Dick. " Now it is all clear to me. The sun shines on the rock, the sand, the trees, the meadow and the pond, and they give back the heat to the air. That is why it is warmer near the earth's surface than it is up among the clouds ; and that is also why the valley is cold at night."
" I knew that you could tell me, if you tried," rippled the brook, gleefully. " But there is some-thing else that I wish you to learn. Bunny may dig a small thin sod out of the meadow, in the shadow of the thick willow, where the grass is short and dry. While he is gone, Dick may fill this pretty shell with cool water from the spring."
The nimble creatures soon, came back with their loads, and laid them in the warm sunshine, on the sandy beach. Then they brought a cold flat stone, and some cool sand which they dug out of the bank, and placed them beside the sod and water.
" Try to find out which will become warm first," was all the brooklet said.
In a few minutes Chip called out, " The rock is warm already, and the sand is still warmer. The sod and water are cold yet."
Wait a little longer," bubbled the brook.
Soon the watchful chipmonk sang out, " The grassy side of the sod is quite warm now, and the water is not so cold as it was. The sand is hot, and the rock is very warm."
" That will do for the sunlight," rippled the little stream. " Now you may bring the stone, sod, water and sand to my thick button-bush, and place them in its cool shadow. We will learn which will give off its heat soonest."
They had not waited long, when the same merry chirping voice was heard, telling them that the sand was cooling very fast, and the rock a little more slowly. But it was quite a while before Chip felt any change in the other two. At length he found that the sod was a little cooler than the water. Then the brooklet was ready to tell them more about how the earth heats the atmosphere.
" Nearly the whole surface of our valley is covered with grass, sand, rocks, trees and water, upon which the sun shines every pleasant day.
You have just found that all parts of this surface are not warmed equally fast. The sand and rocks heat quickly, while the trees and grass-lands warm slowly, and the water still more so.
You know, also, that sand and rocks give off their heat sooner than trees, grass or water. That is why the air above our sandy field is warmer than it is over the pond, when the sun is shining.
"But cool air will flow under and float warm air, and so we find a breeze often flowing from our pond towards the sandy field in the daytime. We call this a ' sea-breeze.'
" At night, long after the sand has given its heat to the atmosphere, the water still warms the air above it. Then a gentle wind flows from over the fields towards the warm pond, and makes a land-breeze.'
"Now you know why the atmosphere is always in motion. The winds are flowing away from cool surfaces towards warmer ones."
"But some winds flow across our valley, over the pasture, water and everything else. Even the hot sand does not turn them aside," said Dick.
" Our sand-pit is very small," replied the brook, " and it heats but little air. In some places there are miles and miles of hot dry surface. The cool atmosphere often flows towards them with great speed and in large quantity.
" Or, if a great deal of vapor rises, it makes the atmosphere lighter, and then the dry winds sweep in with such force that small heated spots, like our pasture and sand-pit, do not stop or bend them from their course. It is only when the atmosphere is almost calm that our pond and field can set the gentle breezes flowing in our valley.
" And who has not listened to the many-sounding winds, sighing in the tall meadow-grasses, whistling up the hillside, or moaning on the edge of the forest that towers and sways like a dark cliff against a background of scudding clouds, and along whose base the waving grains seem to break like surging billows ?
"'The wind has a language, I would I could learn !