156 Our Surroundings 



High tides are of value in deepening the water over the bars 

 at the entrances of channels or at the mouths o*f streams, thus 

 permitting large vessels to sail through. Tides have a value, also, 

 in causing a back and forth flow of water in bays and inlets. 

 These movements clean out any sewage and prevent unhealthy 

 conditions. Progress also has been made in utilizing the differ- 

 ence in the level of the tides to store up water in inlets at high 

 tide, and to develop power by letting it run out through the proper 

 machinery at low tide. The height of tides varies somewhat 

 with the shape of land formations. In the Bay of Fundy, tides 

 of 60 feet have been recorded. Such tides increase the possi- 

 bilities of tide water as a source of power. 



Causes of the Seasons. The change of seasons on the 

 earth is due principally to the inclination of the earth's axis, the 

 revolution of the earth around the sun, and the earth's distance 

 from the sun. These cause variations in the intensity of the heat 

 and light rays received from the sun. 



Experiment to Show Causes of the Seasons. To show 

 that variations in the intensity of heat and light rays of the 

 sun, resulting in the seasons, are due principally to the inclination 

 of the earth's axis, the rotation and revolution of the earth, and 

 its distance from the sun, use the same apparatus employed to 

 illustrate the earth's motion around the sun. 



Remembering that the axis of the earth inclines from the 

 vertical 23^2 degrees, place the globe at A with its axis pointing 

 along the long axis of the ellipse, the top toward the center. 

 Turn the globe on its own axis to represent the daily motion of 

 the earth. Note that during a complete rotation on its axis its 

 whole surface, at one time or another, receives rays from the sun 

 with the exception of the south frigid zone. When the earth 

 is at point A, the sun's rays fall vertically on the tropic of 

 Cancer. The northern hemisphere as a whole has its warmest 

 weather at this time. This is due to the sun's rays falling more 

 directly on the northern hemisphere than on the southern 

 hemisphere. 



Move the globe to point B. Note that the sun's rays fall 

 vertically on the equator and extend from the north to the south 



