SrfENCE OF COMMON THINGS. 59 



What are tides ? Cause of tides. High and low tides. 



The water would become stagnant. 



352 Will waier contaminated with animal and vegetdbk matter under 

 some circumstances purify itself f 



Water contaminated with animal and vegetable mat- 

 ter, if kept for some time, undergoes a spontaneous 

 purification, losing its offensive odor and color, and 

 depositing more or less sediment. "Water, for the sup- 

 ply of ships, is well known to undergo this process of 

 purification by fermentation ; and the larger the quan- 

 tity of destructible matter suspended in it, the more 

 complete and rapid is its purification. 



353 What is a tide f 



A tide is a wave of tJie whole ocean, which is elevated 

 to a certain height, and then sinks, after the manner 

 of a common wave. 



354 What te the cause of tides f 



The attraction of the sun and moon upon the waters 

 of the ocean. The moon being nearest to the earth, 

 her attraction is six times greater, than that of the sun. 

 This attraction of the moon raises the waters of the 

 ocean as they come under her influence by the motion 

 of the earth on its axis. 



355 How many tides are there in a day f 



Two in every lunar day a period of 24 hours 49 



minutes. 



350 What tides are the highest f 



The spring tides. 



357* Why are they higher than at other periods ? 



Because the sun and moon are then in such a posi- 

 tion that they exert their influence together. For every 

 five feet of height in tide produced by the moon, the 

 influence of the sun adds one foot. 



358 What are neap tides t 



Low tid.es. 



35Q Why are neap tides lower than other tides f 



Because then the sun and moon have such positions 

 that their attractive influence is opposed to each other ; 



