How to Do Chemical Tricks — The Barometer, A Novel Barometer
The Barometer.
Now you shall learn something about the pressure exercised by the atmospheric layer which surrounds the earth to the height of about forty miles.
This is done with the aid of a very well-known instrument called the barometer.
You may construct one yourselves.
Procure a glass tube closed at one end, about a yard long and one tenth of an inch in diameter.
Fill it with mercury, then turn it upside down into a bowl filled with the same metal, taking care that the air does not enter the tube.
The column will stop at a height between 29 and 30 inches.
This, therefore is the measure of the force of the air’s pressure, for in the upper part of the tube there is an absolute vacuum and nothing would prevent the mercury from going higher up.
The weight of the air layer corresponds, therefore, to a height of nearly 30 inches of mercury.
[Illustration] This weight has been before stated, viz.
, fourteen and a half pounds, such a weight being supported by every single square inch of the globe’s surface.
A marvelous pressure is thus exerted on the whole earth.
In other words, the weight of the air that surrounds the earth on all sides is no less than the following enormous number of 5,184,740,000,000,000 tons.
A man of average height, himself supports the enormous pressure of 34,171 pounds, or over 15 tons, and yet does not feel the least inconvenience in his movements.
It is because this pressure is exercised in all directions, and a human body carries within it elastic fluids that counterbalance that tremendous weight.
So accustomed do people become to this weight that when the weather is stormy, a feeling of heaviness comes on.
However, it is just the contrary which takes place when the barometer is lower; that is to say, the atmospheric pressure has diminished.
Consequently there is less weight to be carried.
You would experience the same sensation when going up in a balloon.
As you rise higher and higher the weight of the air is less felt, and this makes people so uncomfortable that at a height of about 9,000 or 10,000 yards the liquids in our body--the blood, the water, the bile--tend to escape outwards.
Why?
Because they are no longer balanced by an outside pressure equal in force to them.
In fact, if you continued to ascend, your fate would be that of the bladder in the first experiment--you would burst.
Thus are you and all creatures attached to the face of the earth, and it seems as if great heights were forbidden to our curiosity.
A Novel Barometer.
Construct a toy house of cardboard, painted, and let there be two open doorways in the front, and let it stand on a wooden platform to represent the ground.
The two sides and back may come right down to the ground, but there must be a slight space between the front of the house and the ground upon which it stands.
Next make a flat wheel or disc of wood about the thickness of a penny, its diameter or measurement across the center to measure the same as the distance between the two doorways of the house.
The wheel disc or turn-table must have a shaft or spindle in the middle, so that it will revolve easily in a hole made for it in the floor or ground which your cardboard house stands on; this pivot-hole should be just within the house and exactly half way between the two doors.
In the next place get two small dolls of such size that they will pass easily through the doorways, or you may cut them out of cork or some light substance.
Dress one to represent an old man and the other as his wife, and fix them opposite each other at the edge of the disc or wheel in such a manner, that when it turns on its axle, the figures move in and out of the two doorways provided for their accommodation, for it appears that, although residing in the same house, they are not on very good terms.
When the husband goes out the wife remains at home, and as she only ventures abroad in fine weather, her spouse is obliged to look out when rain may be expected.
The motive power has now to be provided and this takes the form of a piece of catgut, such as violin strings are made of; this is a substance very susceptible of atmospheric influences, for dry weather contracts or tightens it, while a damp atmosphere causes it to relax.
Double your catgut and twist it, fasten one end of the rope so formed near the back of the house inside and fasten the other to the pivot or axle, with two or three turns round it.
As the weather changes the tightening or relaxing of the rope will cause the figures to move in and out of the house.
Of course, the figures must be arranged so that the lady comes out when the rope is tightened by the dryness of the atmosphere.
