60 SCIENCE OF COMMON THINGS. 



Tide movements. Ebb a d flow. Sea waves. 



and for every six feet of the moon's tide, the opposite 

 attraction of the sun takes away one foot. 



36O How fast does the tide wave move f 



The rate of movement of the tide wave depends 

 upon the nature and depth of the sea bottom. With a 

 depth of one fathom, its rate is eight miles per hour ; 

 and with one hundred fathoms, eighty miles per hour. 



\ 361 Does the height of the same tide vary in different places f 



The height of the tide in different places depends 

 much on the configuration of the land ; the same tide 

 may rise in one place three inches, and in another 

 place thirty feet. 



36S At what period during the day is it high water t 



"When the moon passes the meridian that is, when 

 it is nearly vertical over the place the sea is elevated 

 to the greatest extent, and it is said to be high water. 



363 When is it low water f 



"When the moon is upon the horizon, or about six 

 hours after high tide. As the moon passes the meri- 

 dian below the horizon, another elevation occurs, so that 

 we have the ebb and flow of the tide twice every day. 



364 How much later does the tidal eHb and flow occur each day f 



The time becomes later every day by about fifty 

 and a half minutes, which is the excess of a lunar day 

 above a solar one: 28-J- minutes of the former being 

 equal to 27^ minutes of the latter. 



365 What is the cause of ordinary sea waves f 



The wind, pressing unequally on the surface of the 

 sea, depresses one part more than another ; every de- 

 pression causes a corresponding elevation, and these 

 undulations are called waves. 



It must be remembered that waves have no other than a vertical m,o- 

 tion, i.e. up and down. Any substance, as a buoy, floating on a wave, 

 is merely elevated and depressed alternately; it does not otherwise 

 change its place. 



366 If waves are stationary, and only move up anddmon, why do they 

 seem to advance towards the shore f 



This is an ocular deception. When a corkscrew is 



