unequal distance of the tropical belts of high pressure from the equator. These questions may be considered together.

It is to be remembered that the southern hemisphere is the water hemisphere, and that the prevailing westerlies, in gliding over the smooth water surface, are but little retarded by friction and, therefore, attain a higher velocity than the corresponding winds of the northern hemisphere, where the rougher surface materially retards their movement. As a consequence, the circumpolar whirl of the southern hemisphere is stronger, and develops a greater centrifugal force, thus holding a larger quantity of air away from the south pole and reducing the pressure to a greater degree than is brought about by the weaker winds of the northern hemisphere.

Since the circumpolar whirl of the southern hemisphere is the stronger of the two, it withdraws the air to a greater distance from the pole than does its weaker counterpart of the northern hemisphere, and piles it up in the tropical belt of high pressure about five degrees nearer the equator than does the weaker forces of the northern hemisphere.

STORMS

Having gained a comprehensive view of the general, planetary wind system, we may now undertake the study of local disturbances that arise within the general circulation and are known as "storms.'

Storms are simply eddies in the atmosphere. They may be compared to the eddies that are often seen floating along with the current of a river or creek. In these eddies the water is seen to move rapidly around a central vertex, developing sufficient centrifugal force to hold some of the water away from the center, thus forming a well marked depression, frequently of considerable depth. The whole circulation of the eddy is quite independent of the current of the stream which carries it along its course, and while its general direction and velocity of movement coincide with that of the current, there are times when it will be seen to move quickly from side to side and again when it will remain nearly stationary for a time or take on a rapid movement.

The eddies or storms in the atmosphere act in much the same way. They are carried along by the general currents of the river of air in which they exist. Their general direction coincides with the direction of the current in which they are floating, and their rate of movement conforms in a general way to its velocity; but like the eddies in the river, they do not always move in straight lines nor at a uniform rate of speed.

There is one important respect in which the eddies in the air differ from eddies in water. The water eddy may revolve in either direction, depending upon the direction in which the initial force was applied, but the storm eddies in the atmosphere always revolve counter-clockwise in the northern hemisphere, and clockwise in the southern.

This is due to the deflecting force of the earth's rotation, which is fully explained on page 872.

WEATHER MAPS

A weather map is a sort of flashlight photograph of a section of the bottom of one or more of these great rivers of air. It brings into view the whole meteorological situation over a large territory at a given instant of time; and, while a single map conveys no indication of the movements continually taking place in the atmosphere, a series of maps, like a moving picture, shows not only the whirling eddies, the hurrying clouds and the fast-moving winds, but the ceaseless on-flow of the great river of air in which they float. Our present knowledge of the movements of the atmosphere has been gained chiefly from a study of weather maps; they form the basis of the modern system of weather forecasting, and their careful study is essential to any adequate understanding of the problems presented by the atmosphere. (See pages 884-885.)

Snow crystal.
Photomicrograph by
W. A. Bentley.

The Principles of Weather Forecasting

The forecasting of the weather has been made possible by the electric telegraph. It is based upon a perfectly simple, rational process constantly employed in everyday affairs. We go to a railway station and ask the operator about a certain train. He tells us that it will arrive in an hour. We accept his statement without question, because we are confident that he knows the speed at which the train is approaching, a few clicks of his telegraph instrument has told him just where it is and the time it will arrive, barring accidents, is a simple calculation. Information of coming weather changes are obtained in a similar manner. Although storms do not run on steel rails like a train, nevertheless their movements may be foreseen with a reasonable degree of accuracy, depending chiefly upon the size of the territory from which telegraphic reports are received and the experience and skill of the forecaster. As a rule, the larger the territory brought under observation, especially in its longitudinal extent (the general currents carry storms of the middle latitudes eastward around the world and those of the tropics westward), the earlier advancing changes may be recognized and the more accurately their movements foreseen.

Forecasts Based on Weather Maps

The forecasts issued by the United States Weather Bureau are based on weather maps, prepared from observations taken at 8 a. m. and 8 p. m. at about 200 observatories. In addition to the reports received by telegraph by the Central Office at Washington, the several forecast centers and other designated stations from observatories or stations in the United States, a system of interchange with Canada, Mexico, the West Indies and other island outposts in the Atlantic and Pacific gives to the forecaster two daily photographs of the weather conditions over a territory embracing nearly

the whole of the inhabited part of the western hemisphere north of the equator. Any sort of disturbance within this vast region is photographed at once upon the weather map. If it be a West Indies hurricane or other destructive storm, its character is recognized instantly, its rate and direction determined and information of the probable time of its arrival sent to those places that lie in its path. The method is perfectly simple. Anyone with a weather map and a little experience can forecast the weather with some degree of accuracy, or, at least, gain an intelligent understanding of the conditions upon which the forecasts that accompany the map are based.

Maps, Where Published and How Obtained

Weather maps are published in many daily papers, and in somewhat larger form and more in detail, at many Weather Bureau stations. They may usually be obtained for school use by applying to the nearest Weather Bureau station or to the Chief of the Weather Bureau at Washington, D. C.

The forecasts that accompany the maps are simply an expression on the part of the official forecaster as to the weather changes he expects to occur in various parts of the country within the time specified, usually within 36 to 48 hours. His opinion is based upon the conditions shown by the map. He has no secret source of information. You may accept his conclusions, or, if in your opinion they are not justified, you have all the information necessary to make a forecast for yourself. Weather maps are published so extensively with a view to thus stimulating an intelligent interest in the problem of weather forecasting, and also that one may see at a glance what the temperature, rainfall, wind and weather is in any part of the country in which he may be interested. The friends of the weather service are those who best understand its work.

THE VALUE OF THE WEATHER SERVICE

No one knows so well as the forecaster that the changes that appear most certain to come sometimes fail, or come too late; but taking all in all, about 85 out of 100 forecasts are correct. Of those that fail, probably not more than three of four per cent. fail because the changes come unannounced. Most forecasters predict too much, and their forecasts fail because the expected changes come after the time specified or not at all. It is fortunate that this is so; for it is better to be prepared for the change though it be late in coming than to have it come without warning.

The value of the weather service to the agriculture and commerce of the United States cannot be questioned seriously. That the appropriations for its support have been increased year by year from $1,500 in 1871 to nearly $1,500,000 in 1910 is evidence of its value and efficiency. A conservative estimate places the value of property saved by the warnings issued by the Weather Bureau at $30,000,000 annually.

LESSON CCXXII

EXPERIMENTS TO SHOW AIR PRESSURE

Leading thought-The air presses equally in all directions.

Experiment 1-To show that air presses upward-Fill a tumbler which has an unbroken edge as full of water as possible. Take a piece of writing paper and cover the tumbler, pressing the paper down firmly upon the edge of the glass. Turn the glass bottom side up and ask why the water does not flow out. Allow a little air to enter; what happens? Why? Turn the glass filled with water and covered with paper sidewise; does the water flow out? If not, why?

Experiment 2-To show that air passes downward-Ask some of the boys of the class to make what they call a sucker. This is a piece of leather a few inches across. Through its center a string is drawn which fits very closely into the leather and is held in place by a very flat knot on the lower side. Dampen the leather and press it against any flat surface, and try to pull it off. If possible, place the sucker on a flat stone and see how heavy a stone can be lifted by the sucker. Ask why a sucker clings so to the flat surface. If a little air is allowed to get between the sucker and the stone, what happens? Why?

Hints to the teacher regarding the Experiments-The water is kept in the tumbler in Experiment 1 by the pressure of the atmosphere against the paper. If the tumbler is tipped to one side the water still remains in the glass, which shows that the air is pressing against the paper from the side with sufficient force to restrain the water, and if the tumbler is tipped bottom side up it shows the air is pressing upward with sufficient force to keep the water within the glass.

In the case of Experiment 2, we know that the leather pressing upon the floor or on the stone is not in itself adhesive, but it is made wet simply so that it shall press against the smooth surface more closely. The reason why we cannot pull it off is that the air is pressing down upon it with the force of about fifteen pounds to the square inch. If the experiment is performed at sea level, we should be able to lift by the string of the sucker a stone weighing fifteen pounds. The reason why the water falls out of the tumbler after a little air is let beneath the paper, is that then the air is pressing on both sides of the paper; and the reason why the sucker will not hold if there is any air between it and the stone, is because the air is pressing in both directions upon it.

Supplementary reading-The Wonderbook of the Atmosphere, Houston, Chapters III, IV, V.

LESSON CCXXIII
THE BAROMETER

Leading thought-The weight of our atmosphere balances a column of mercury about thirty inches high, and is equal to about fifteen pounds to the square inch. This pressure varies from day to day, and becomes less as the height of the place increases. The barometer is an instrument for measuring the atmospheric pres sure. It is used in finding the height of mountains, and, to a certain extent, it indicates changes of the weather.

Method-A glass tube about 36 inches long, closed at one end; a little glass funnel about an inch in diameter at the top; a small cup a bird's bathtub is a good size since it allows plenty of room for the fingers; mercury enough to fill the tube and more deep in the cup. Be careful not to spill the mercury in the following process, or you will be as badly off as old Sisyphus with his rolling stone.

have the mercury an inch or

Set the closed end of the tube in the cup so that any spilled mercury will not be lost; with the help of the funnel slowly and carefully fill the tube clear to the top with the mercury; empty the rest of the mercury into the cup; place the end of one of the fingers of the left hand tightly over the open end of the tube and keep it there; with the right hand invert the tube, keeping the end closed with the finger, and place the hand, finger and all, beneath the mercury in the cup then remove the finger, keeping the open end of the tube all the time below the surface of the mercury. When the mercury has ceased to fall measure the distance from the surface in the cup to the top of the mercury in the tube.

Observations-1. How high is the column of mercury in the tube?

2.

What keeps the mercury in the tube? Place the cup and the tube on a table in the corner of the room, place behind the tube a yardstick, and note whether the column of mercury is the same height day after day. If it varies, why?

A barometer made by pupils.

3. Would the mercury column be as high in the tube if it were placed on top of a mountain as it would at the foot? Why?

Supplementary reading-Chap. II in The Wonderbook of the Atmos phere, Houston.