142 



FIRST TEAR SCIENCE 



areas and the high-temperature areas are over the land. 

 This illustrates what we have already learned; that land 

 heats and cools much more rapidly than water and that 

 hot air is lighter than cold air. 



In both summer and winter there is an area of compar- 

 atively high pressure on either side of the equator, but this 

 area is not fixed; it moves north and south. In summer 

 it is farthest north in the northern hemisphere. 



The winds are simply a transfer of air from a place 

 where the pressure is high to a place where it is low, or a 

 transfer of air along what are called barometric gradients 

 from a high barometer to a low barometer. So the 

 above-mentioned changes in the relation between the pres- 

 sure on the land and on the sea must have an effect upon 

 the directions of the winds. As a rule the wind blows out 

 from the land interiors in the winter and into these inte- 

 riors in the summer. It is thus seen that isotherms and 

 isobars are closely related to each other, and that the wind 

 is but a result of the atmospheric conditions which they 

 represent. 



70. Wind. Experiment 77. On a day when the temperature in 

 the room is considerably higher than that outside, open a window at 

 the top and bottom and -hold a strip of tissue paper in front of the 

 opening. Is there an air current, and if so, in what direction does it 



move at the top and at the bottom of 

 the window ? What causes " drafts " 

 in a room ? 



Experiment 78. Procure two sim- 

 ilar dishes about 15 cm. high and 5 or 

 6 cm. in diameter with short tubes 

 of about 1 cm. in diameter opening 

 Fig. 69. out from near the top and bottom. 



Connect the bottom tubes of the two 



dishes with a tightly fitting rubber tube. Do the same with 'the top 

 tubes. Place a Hoffman's screw upon each of the rubber tubes and 

 screw it tight so that no liquid can flow through either tube. Fill 



