Experiment 68. -Fill a bottle with clean water and fit it tightly with a rubber stopper having two holes in it. Plug one of the holes tightly with a glass tube one end of which has been closed by heating in a Bunsen burner. Through the other hole put an openFig. 58.
Fig. 59.
glass tube 10 to 15 cm. long. See that both tubes fit tightly in the stopper and that the stopper fits tightly in the bottle. Now attempt to "suck" the water out of the bottle through the open tube. Does it come out freely? Pull out the glass plug. Does it come out any better? If so, why?
The mercurial barometer we have already made in a rough form. The best form of these instruments consists of a glass tube of uniform bore about eighty centimeters long and closed at one end. After being carefully filled with pure mercury, it is inverted in a cistern of mercury. The cistern of mercury has a sliding bottom easily moved up and down by means of a set screw. At the top of the cistern there is a short ivory peg. The lower end of the ivory peg is at anexactly measured distance from the bottom of a scale. The scale is placed beside a slit near the top of a metallic tube which is firmly fastened to the cistern and surrounds and protects the glass tube.
When it is desired to read the barometer, the sliding bottom of the cistern is raisedor lowered until the top of the mercury
Fig. 60.
in the cistern just touches the bottom of the ivory peg. The height of the top of the mercury column is then read from the scale. In order to determine the height with great precision there is generally attached to the metallic tube a sliding vernier which moves in the slit.
The aneroid barometer consists in general of a corrugated metallic box from which the air has been partially exhausted. Within the box is a stiff spring so that the pressure of the air will not cause it to collapse. Attached to the box are levers by which any changeBAROGRAPH.
This is arranged so as to record the
air pressure automatically for a week at a time.
in the volume of the box will be multiplied and indicated by apointer arranged to move over a dial. The dial has a scale upon it and thus the air pressure is registered.
Instruments called barographs are constructed in which a longlever provided with a pen point is attached to the aneroid and made to record on a cylinder revolved by clockwork. Thus a continual record is made of barometric readings.
58.Determination of Height by a Barometer.
Experiment 69. -Carry an aneroid barometer from the bottom of a high building to the top. Note the reading of the barometer at the bottom and again at the top. Why is the barometer lower at the top of the building?
As the pressure of air at any surface is due to the weight of the air above that surface, it happens that as we go up the pressure decreases, since there is a continually decreasing weight of air above. If the rate of this decrease is determined, then it is possible to determine the elevation by ascertaining the pressure.
Although the height of the barometer is continually varying with the changing air conditions, yet if these conditions remain about the same, it may roughly be estimated that the fall of 1/16 of an inch in the height of the mercury column indicates a rise of about 57 feet, and that the fall of a millimeter indicates a rise of about 11 meters. These values are fairly reliable for elevations less than a thousand feet, under ordinary temperatures and pressures.
At the height of 25 miles the barometric column would probably not be more than 1/25 of an inch high. Several measurements made in different ways indicate that the air is at least 100 miles in depth, probably more. Nearly three fourths of the atmosphere however is below the top of the highest mountain. The highest altitude ever reached by man was about 7 miles.
To study air conditions small balloons to which meteorological instruments are attached have been sent to a height of 21 miles. It is found that the minimum temperatures occur at a height of from 6 to10 miles. Conditions affecting weather, however, seem to extend to a height of not much over 3 miles.
59.Adiabatic Heating and Cooling of Air.
Experiment 70. -Have a five-pint glass bottle fitted with a two- hole rubber stopper. Pass through the holes in the stopper a chemical or air thermometer and a short glass tube the lower endof which extends into the bottle not near the bulb of thethermometer, so that when the air is exhausted or allowed to enter the bottle there will be no movement of the air near the bulb of the thermometer. The end of the column of the thermometer must be visible above the stopper.
Attach the glass tube to an air pump by means of a thick-walled rubber tube. Note the temperature of the thermometerFig. 61.
within the bottle and also of the air outside. Quickly exhaust the air from the bottle, carefully noting the action of the thermometer. See that the temperature of the air in the room does not change during the experiment. Allow the air quickly to enter the bottle and note the action of the thermometer. The temperature inside the bottle changes as the air is quickly exhausted, or as it is allowed to enter the bottle again and thus to increase the density of the air in the bottle.
It has been found that when air expands its temperature falls and when it is compressed its temperature rises. This heating and cooling of the air without the application of external heat or cold, but simply by a change in the density of the gas itself, is called adiabatic heating or cooling. It is taken advantage of in the manufacture of liquid air and is the same principle which is utilized in cold storage plants. This property of air has much to do in developing our wind circulation and storms.
