160 FIRST TEAR SCIENCE 



resorted to if agriculture is to succeed. The fall of rain 

 on the mountains and high plateaus supplies rivers of 

 sufficient size to furnish water for extensive irrigation, 

 and so a considerable part of the area which is now prac- 

 tically a desert will in the future be reclaimed for the use 

 of man. The government is at present engaged in ex- 

 tensive irrigation work in this territory. 



From about the 100th meridian to the Atlantic Ocean 

 there is a varying rainfall, but it is as a rule sufficient for 

 the needs of agriculture. It gradually increases toward 

 the east, moisture being supplied plentifully from the Gulf 

 of Mexico and the Atlantic Ocean by the southerly and 

 easterly winds. The rainfall is well distributed through- 

 out the year and averages from thirty to sixty inches. 



79. Electricity. Experiment 82. Place some small pieces of 

 paper or pith balls on a table and after rubbing a glass rod with silk 

 bring it near the pieces. Do the same with a stick of sealing wax or 

 a hard-rubber rod rubbed with flannel or a cat's skin. Note the 

 action of the pieces. 



Experiment 83. Rub a glass rod briskly with silk and place in a 

 wire sling such as was used in Experiment 12. Bring toward one 

 end of the glass rod another glass rod which has been rubbed with 

 silk. Do the rods attract or repel each other? Bring toward the 

 suspended rod a piece of sealing wax or a vulcanite rod which has 

 been rubbed with flannel or a cat's skin. Does this repel or attract 

 the glass rod? 



Experiment 84. Suspend a pith ball by a silk thread from the 

 ring of a ring stand. Rub a glass rod with a piece of silk and bring 

 it near the pith ball but do not allow the two to touch. Note the 

 action of the ball. "Touch the pith ball with the rod. Does it behave 

 now as it did before ? Rub a vulcanite rod with a piece of flannel or 

 cat's skin and bring it near a suspended pith ball. Does the pith ball 

 act as it did with the glass rod ? Touch the pith ball with the rod. 

 How does it act ? Bring a glass rod rubbed with silk near a pith ball 

 which has been in contact with a vulcanite rod after it was rubbed 

 with flannel or a cat's skin. Does the glass rod repel or attract the 

 ball? 



