80 LABORATORY EXERCISES 



thermometer until dew is deposited. Get the thermometer 

 reading; is it the same as before? 



b. If air is saturated with water vapor, and we lower its 

 temperature, even slightly, we cause a fog to be formed. One 

 way by which we can cool air is to cause it to expand. The 

 experiment is carried out by pouring water in spurts (cf. 

 Exercise 1) from a large bottle that has a small mouth. Half 

 fill such a bottle with water, and invert it over a pail or sink. 

 Notice that as a spurt of water falls out, and the air inside is 

 slightly expanded, a cloud or fog is formed, but that as soon as 

 an air bubble enters, and the air in the bottle is again at the 

 ordinary pressure, the fog disappears. 



How does this experiment explain the formation of clouds in 

 rising currents of air? 



EXERCISE 77 

 WEATHER RECORDS 



Apparatus and Materials. Weather reports, or thermometer, 

 barometer, rain gauge, and weather vane; if possible, an anemometer. 



a. Keep a weather record for at least two weeks, putting 

 down your observations under the heads given below. For 

 the temperature you can use your home thermometer or the 

 one at school. For the barometer height inquire each day at 

 school, or get it from the daily weather report in your local 

 newspaper. You will need to depend on some one with a rain 

 gauge, or on the weather report, for the amount of precipitation; 

 you yourself can tell whether it is rain or snow. 



You can also tell the direction of the wind. Judge of its 

 velocity according to the table given in 274 of the text. 

 Describe the clouds as in 269 of the text. Make your own 

 readings, twice a day if possible: first in the forenoon, and again 

 in the afternoon. 



