i. How is rain or snow shown for large areas?

j. How many "Lows" are on the map? What is the air pressure in each ?

k. How many "Highs" are on the map? What is the pressure in each ?

7. Where are the local forecasts found on the map ?

m. Find the pressure, temperature, wind direction, and relative humidity for your locality as given for the early morning hour.

2. What is the relation between air pressure and wind? Turn to page 39 (Fig. 24) of the text. With a compass draw a circle having a radius of one inch using the "o" of the word "Low" as a center. Count the number of arrows inside of or on the circle. Record the number in the table.

3. Place a thin sheet of paper over figure 24 of the text. Trace the outline of the circle and the word "Low." Make a dot in the circle over every arrow which points in a direction opposite that in which the hands of a clock move. Count the dots and record. 4. Repeat 3, but count only the arrows which point inward toward the "Low." Record the number.

Questions. What is the general direction of the circulation of the air about a low-pressure area? Is the general movement of air toward this area or away from it?

Suggestions for report. Record the figures in this table.

Reference work. Read sections 42 to 54. What are the usual paths of low-pressure areas across the United States?

Optional problems. Try to forecast the weather every day for one week. Try the experiment suggested in figure 27 of the text.

DANGEROUS AND BENEFICIAL MIXTURES OF GASES (V−1)

The problem. It is a familiar fact that any pronounced odor, such as that of ammonia, gasoline, benzine, or illuminating gas, is readily detected in all parts of a room soon after it is released. Usually the odor will disappear in a short time, but we seldom raise the question as to the cause of the spread of the odor or of its disappearance. Mixtures of air and gasoline or of air and illuminating gas when ignited sometimes cause explosions with fatal results. Explosions of mine gases continue to endanger the lives of miners in spite of constant efforts to obtain safety by means of special devices and precautions. In all of these cases mixtures of gases are involved. How do gases mix with one another?

What to use. Two wide-mouth bottles, one glass plate or card two inches square, matches, rubber tube two feet long, a sink or basin with water in it, and illuminating gas, or some gas-forming substance. If illuminating gas is not available ammonia gas from household ammonia water may be used.

FIG. 12

What to do. 1. With pupils seated at their desks release the gas, noting the exact time of doing so. Each pupil should record the time when he first detects the gas. Open the windows to remove the gas.

2. Attach a rubber tube to a gas jet. Fill a bottle level full of water, place a glass plate or card over the top, hold it firmly, and invert it in the sink or basin of water so that all of the water remains in the bottle. The glass plate or card may now be removed. Place one end of the rubber tube under the mouth of the bottle and let the gas slowly fill the bottle. While it is under water close the mouth of the bottle with the plate or card. Bring the bottle,

mouth downward, and place it on a similar bottle filled with air (Fig. 12). Withdraw the plate and let the bottle stand fifteen minutes. Test the contents of both bottles with a burning match.

3. Repeat 2, but place the bottle containing air above the bottle containing illuminating gas.

Questions. How does the gas which was released reach all parts of the room? Is the room now "full of gas"? Why is the bottle held mouth downward? Did the heavy gas rise? Why? Did the light gas enter the lower bottle? Why? What facts are shown by the experiment?

Suggestions for report. Write a complete description of 1 or 2, telling what was done, what occurred, and why it occurred as it did.

Reference work. Read Chapter V of the text. Secure a report of a case in which illuminating gas caused fatal effects, and explain how it occurred. How should artificial respiration be used in cases of injury from harmful gases?

Optional problems. Since the gases of the air have different weights, why do they not form layers? Can you show how to pour a gas from one bottle to another? Devise an experiment to show the mixing of heavy and light liquids. Perform the experiment suggested at the bottom of page 62 of the text, using equal quantities of alcohol and water.