SCIENCE OF COMMON THINGS. 61 



Breakers. Spray of waves. Swr 



turned round, the thread appears to move forward ; 

 and the apparent onward motion of the waves of the 

 sea is a similar delusion. 



367 What is the cause of breakers ? 



The interference of rocks or rising ~banks^ in the sea 

 with the regular form of the wave, by which the out> 

 line or curve of the wave is broken. 



368 What causes the spray of waves f 



The wind driving the surface of the water from the 

 top of the wave, and scattering the small particles in 

 all directions. 



369 What is the surf? 



When the shore runs out very shallow for a great 

 extent, the breakers are distinguished by the name of 

 surf. 



SVO What do we know concerning the magnitude and velocity of ocean 

 waves ? 



On the Atlantic, during a storm, the waves rise to a 

 height of about forty-three feet above the hollow occu- 

 pied by the ship ; the total distance between the crests 

 of two large waves being 559 feet, which distance is 

 passed by the wave in about seventeen seconds of time. 



371 With what velocity is it estimated that such storm waves as the 

 above described travel f 



At the rate of about thirty-two miles per hour. 



A wave is a /orw, and not a thing ; the form advances, but not the 

 substance of the waves. 



3753 If a cock at the extremity of a pipe be suddenly closed while water 

 is running through, why is a noise and shock produced ? 



Because the forward motion of the whole body of 

 the water contained in the pipe being instantly arrested, 

 and the momentum of a liquid being as great as that 

 of a solid, the water strikes the cock with as much force 

 as if it were a long bar of metal, or a rod of wood hav- 

 ing the same weight and velocity as the water. Then, 

 as a fluid presses equally in all directions, a leaden 

 pipe of great length may be widened, or even burst in 

 the experiment. 



