FOOD FOR PLANTS 199 



tropics; and 9,000 to 4,000 between the latitudes of 

 forty and forty-nine degrees. 



At a depth of forty-five feet under ground, the tem- 

 perature of the earth is uniform throughout the year. 



In summer time, the season of ripening moves 

 northward at the rate of about ten miles a day. 



The human ear is so extremely sensitive that it can 

 hear a sound that lasts only the twenty-four thou- 

 sandth part of a second. Deaf persons have some- 

 times conversed together through rods of wood held 

 between their teeth, or held to their throat or breast. 



The ordinary pressure of the atmosphere on the 

 surface of the earth is two thousand one hundred and 

 sixty pounds to each square foot, or fifteen pounds to 

 each square inch ; equal to thirty perpendicular inches 

 of mercury, or thirty-four and a half feet of water. 



Sound travels at the rate of one thousand one hun- 

 dred and forty-two feet per second — about thirteen 

 miles in a minute. So that if we hear a clap of 

 thunder half a minute after the flash, we may calcu- 

 late that the discharge of electricity is six and a half 

 miles off. 



^ Lightning can be seen by reflection at the distance 

 of two hundred miles. 



The explosive force of closely confined gunpowder 

 is six and a half tons to the square inch. 



Facts for the Weatherwise. 



If the full moon rises clear, expect fine weather. 



A large ring around the moon and low clouds in- 

 dicate rain in twenty-four hours; a small ring and 

 high clouds, rain in several days. 



