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Simple Science

81. Use of the Barometer
Changes in air pressure are very closely connected with changes in the weather. The barometer does not directly foretell the weather, but a low or falling pressure, accompanied by a simultaneous fall of the mercury, usually precedes foul weather, while a rising pressure, accompanied by a simultaneous rise in the mercury, usually precedes fair weather. The barometer is not an infallible prophet, but it is of great assistance in predicting the general trend of the weather. There are certain changes in the barometer which follow no known laws, and which allow of no safe predictions, but on the other hand, general future conditions for a few days ahead can be fairly accurately determined. Figure shows a barograph or self-registering barometer which automatically registers air pressure.

Seaport towns in particular, but all cities, large or small, and villages too, are on request notified by the United States Weather Bureau ten hours or more in advance, of probable weather conditions, and in this way precautions are taken which annually save millions of dollars and hundreds of lives.

I recollect a summer spent on a New Hampshire farm, and know that an old farmer started his farm hands haying by moonlight at two o'clock in the morning, because the Special Farmer's Weather Forecast of the preceding evening had predicted rain for the following day. His reliance on the weather report was not misplaced, since the storm came with full force at noon. Sailing vessels, yachts, and fishing dories remain within reach of port if the barometer foretells storms.

FIG. - Barograph.
82. Isobaric and Isothermal Lines
If a line were drawn through all points on the surface of the earth having an equal barometric pressure at the same time, such a line would be called an isobar. For example, if the height of barometers in different localities is observed at exactly the same time, and if all the cities and towns which have the same pressure are connected by a line, the curved lines will be called isobars. By the aid of these lines the barometric conditions over a large area can be studied. The Weather Bureau at Washington relies greatly on these isobars for statements concerning local and distant weather forecasts, any shift in isobaric lines showing change in atmospheric pressure.

If a line is drawn through all points on the surface of the earth having the same temperature at the same instant, such a line is called an isotherm.

FIG. - Isotherms.
83. Weather Maps
Scattered over the United States are about 125 Government Weather Stations, at each of which three times a day, at the same instant, accurate observations of the weather are made. These observations, which consist of the reading of barometer and thermometer, the determination of the velocity and direction of the wind, the determination of the humidity and of the amount of rain or snow, are telegraphed to the chief weather official at Washington. From the reports of wind storms, excessive rainfall, hot waves, clearing weather, etc., and their rate of travel, the chief officials predict where the storms, etc., will be at a definite future time. In the United States, the general movement of weather conditions, as indicated by the barometer, is from west to east, and if a certain weather condition prevails in the west, it is probable that it will advance eastward, although with decided modifications. So many influences modify atmospheric conditions that unfailing predictions are impossible, but the Weather Bureau predictions prove true in about eight cases out of ten.

The reports made out at Washington are telegraphed on request to cities in this country, and are frequently published in the daily papers, along with the forecast of the local office. A careful study of these reports enables one to forecast to some extent the probable weather conditions of the day.

The first impression of a weather map with its various lines and signals is apt to be one of confusion, and the temptation comes to abandon the task of finding an underlying plan of the weather. If one will bear in mind a few simple rules, the complexity of the weather map will disappear and a glance at the map will give one information concerning general weather conditions just as a glance at the thermometer in the morning will give some indication of the probable temperature of the day.

On the weather map solid lines represent isobars and dotted lines represent isotherms. The direction of the wind at any point is indicated by an arrow which flies with the wind; and the state of the weather - clear, partly cloudy, cloudy, rain, snow, etc. - is indicated by symbols.

FIG. Weather Map
84. Components of the Air
The best known constituent of the air is oxygen, already familiar to us as the feeder of the fire without and within the body. Almost one fifth of the air which envelops us is made up of the life-giving oxygen. This supply of oxygen in the air is constantly being used up by breathing animals and glowing fires, and unless there were some constant source of additional supply, the quantity of oxygen in the air would soon become insufficient to support animal life. The unfailing constant source of atmospheric oxygen is plant life (Section 48). The leaves of plants absorb carbon dioxide from the air, and break it up into oxygen and carbon. The plant makes use of the carbon but it rejects the oxygen, which passes back into the atmosphere through the pores of the leaves.

Although oxygen constitutes only one fifth of the atmosphere, it is one of the most abundant and widely scattered of all substances. Almost the whole earth, whether it be rich loam, barren clay, or granite boulder, contains oxygen in some form or other; that is, in combination with other substances. But nowhere, except in the air around us, do we find oxygen free and uncombined with other substances.

A less familiar but more abundant constituent of the atmosphere is the nitrogen. Almost four fifths of the air around us is made up of nitrogen. If the atmosphere were composed of oxygen alone, the merest flicker of a match would set the whole world ablaze. The fact that the oxygen of the air is diluted as it were with so large a proportion of nitrogen, prevents fires from sweeping over the world and destroying everything in their path. Nitrogen does not support combustion, and a burning match placed in a corked bottle goes out as soon as it has used up the oxygen in the bottle. The nitrogen in the bottle, not only does not assist the burning of the match, but it acts as a damper to the burning.

Free nitrogen, like oxygen, is a colorless, odorless gas. It is not poisonous; but one would die if surrounded by nitrogen alone, just as one would die if surrounded by water. The vast supply of nitrogen in the atmosphere would be useless if the smaller amount of oxygen were not present to keep the body alive. Nitrogen is so important a factor in daily life that an entire chapter will be devoted to it later.

Another constituent of the air with which we are familiar is carbon dioxide. In pure air, carbon dioxide is present in very small proportion, being continually taken from the air by plants in the manufacture of their food.

Various other substances are present in the air in very minute proportions, but of all the substances in the air, oxygen, nitrogen, and carbon dioxide are the most important.
85. Bicycle Tires
General Properties of Gases:
We know very well that we cannot put more than a certain amount of water in a tube, but we know equally well that the amount of air which can be pumped into a bicycle or automobile tire depends largely upon our muscular energy. A gallon of water remains a gallon of water and requires a perfectly definite amount of space, but air can be compressed and compressed, and made to occupy less and less space. While it is true that air is easily compressed, it is also true that air is elastic and capable of very rapid and easy expansion. If a puncture occurs in a tire, the compressed air escapes very quickly; that is, the compressed air within the tube has taken the first opportunity offered for expansion.

The fact that air is elastic has added materially to the comfort of the world. Transportation by bicycles and automobiles has been greatly facilitated by the use of air tires. In many hospitals, air mattresses are used in place of hair, feather, or cotton mattresses, and in this way the bed is kept fresher and cleaner, and can be moved with less danger of discomfort to the patient. Every time we squeeze the bulb of an atomizer, we force compressed or condensed air through the atomizer, and the condensed air pushes the liquid out of the nozzle. Thus we see that in the necessities and conveniences of life compressed air plays an important part.

FIG. - By squeezing the bulb, air is forced out of the nozzle.
86. The Danger of Compression
General Properties of Gases:
Air under ordinary atmospheric conditions exerts a pressure of 15 pounds to the square inch. If, now, large quantities of air are compressed into a small space, the pressure exerted becomes correspondingly greater. If too much air is blown into a toy balloon, the balloon bursts because it cannot support the great pressure exerted by the compressed air within. What is true of air is true of all gases. Dangerous boiler explosions have occurred because the boiler walls were not strong enough to withstand the pressure of the steam (which is water in the form of gas). The pressure within the boilers of engines is frequently several hundred pounds to the square inch, and such a pressure needs a strong boiler.
87. How Pressure is Measured in Buildings
General Properties of Gases:
In the preceding Section we saw that undue pressure of a gas may cause explosion. It is important, therefore, that authorities keep strict watch on gases confined within pipes and reservoirs, never allowing the pressure to exceed that which the walls of the reservoir will safely bear.

Pressure in a gas pipe may be measured by a simple instrument called the pressure gauge: The gauge consists of a bent glass tube containing mercury, and so made that one end can be fitted to a gas jet. When the gas cock is closed, the mercury stands at the same level in both arms, but when the cock is opened, the gas whose pressure is being measured forces the mercury up the opposite arm. If the pressure of the gas is small, the mercury changes its level but very little. It is clear that the height of a column of mercury is a measure of the gas pressure. Now it is known that one cubic inch of mercury weighs about half a pound. Hence a column of mercury one inch high indicates a pressure of about one half pound to the square inch; a column two inches high indicates a pressure of about one pound to the square inch, and so on.

This is a very convenient way to measure the pressure of the illuminating gas in our homes and offices. The gauge is attached to the gas burner and the pressure is read by means of a scale attached to the gauge.

In order to have satisfactory illumination, the pressure must be strong enough to give a steady, broad flame. If the flame from any gas jet is flickering and weak, it is usually an indication of insufficient pressure and the gas company should investigate conditions and see to it that the consumer receives his proper value.

FIG. - A pressure gauge.
88. The Gas Meter
General Properties of Gases:
Most householders are deeply interested in the actual amount of gas which they consume (gas is charged for according to the number of cubic feet used), and therefore they should be able to read the gas meter which indicates their consumption of gas. Such gas meters are furnished by the companies, and can be read easily.

The instrument itself is somewhat complex. It will suffice to say that within the meter box are thin disks which are moved by the stream of gas that passes them. This movement of the disks is recorded by clockwork devices on a dial face. In this way, the number of cubic feet of gas which pass through the meter is automatically registered.

FIG. - The gas meter indicates the number of cubic feet of gas consumed.
89. The Relation between Pressure and Volume
General Properties of Gases:
It was long known that as the pressure of a gas increases, that is, as it becomes compressed, its volume decreases, but Robert Boyle was the first to determine the exact relation between the volume and the pressure of a gas. He did this in a very simple manner.

Pour mercury into a U-shaped tube until the level of the mercury in the closed end of the tube is the same as the level in the open end. The air in the long arm is pressing upon the mercury in that arm, and is tending to force it up the short arm. The air in the short closed arm is pressing down upon the mercury in that arm and tending to send it up the long arm. Since the mercury is at the same level in the two arms, the pressure in the long arm must be equal to the pressure in the short arm. But the long arm is open, and the pressure in that arm is the pressure of the atmosphere. Therefore the pressure in the short arm must be one atmosphere. Measure the distance bc between the top of the mercury and the closed end of the tube.

Pour more mercury into the open end of the tube, and as the mercury rises higher and higher in the long arm, note carefully the decrease in the volume of the air in the short arm. Pour mercury into the tube until the difference in level bd is just equal to the barometric height, approximately 32 inches. The pressure of the air in the closed end now supports the pressure of one atmosphere, and in addition, a column of mercury equal to another atmosphere. If now the air column in the closed end is measured, its volume will be only one half of its former volume. By doubling the pressure we have reduced the volume one half. Similarly, if the pressure is increased threefold, the volume will be reduced to one third of the original volume.

FIGS. 54, 55. - As the pressure on the gas increases, its volume decreases.
90. Heat due to Compression
General Properties of Gases:
We saw in Section 89 that whenever the pressure exerted upon a gas is increased, the volume of the gas is decreased; and that whenever the pressure upon a gas is decreased, the volume of the gas is increased. If the pressure is changed very slowly, the change in the temperature of the gas is imperceptible; if, however, the pressure is removed suddenly, the temperature falls rapidly, or if the pressure is applied suddenly, the temperature rises rapidly. When bicycle tires are being inflated, the pump becomes hot because of the compression of the air.

The amount of heat resulting from compression is surprisingly large; for example, if a mass of gas at 0° C. is suddenly compressed to one half its original volume, its temperature rises 87° C.

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