42 HARBOUR CONSTRUCTION. 



A paddle-steamship, going at full speed, and suddenly having 

 her engines put at half-speed or dead slow, would, by her 

 momentum, advance for some time at a speed that would cover 

 a greater distance than that represented by the periphery of her 

 wheels multiplied by the number of revolutions made by them. 

 The wheels would, therefore, to a certain extent, be dragged 

 through the water by the vessel's momentum, and, assuming that 

 they touched the soft bottom, they would drag a portion of the 

 material of which it was composed along with them, notwith- 

 standing that they would be revolving in a direction which, 

 under different circumstances, would have caused them to draw 

 the material towards them and cast it out behind. I have 

 often watched rollers coming in after a gale, the wind having 

 suddenly chopped round and blowing strong off-shore at the 

 time. These rollers appeared as in the following sketch, 



< ^ Direction of propagation of wave. 



Direction of gale. 



FIG, 8. Breaker advancing against off-shore gale. 



(Fig. 8), with their crests, so to speak, being dragged off by the 

 opposing wind. It was impossible to say, from observation, how 

 the motion of the revolving particles constituting such waves 

 was being influenced, but might it not be that the period of 

 their revolution was thrown out of beat or harmony with the 

 speed of the wave, the off-shore wind acting as a brake, and 

 the momentum of the waves, meanwhile, enabling them to main- 

 tain their form, notwithstanding that the motion of their particles 

 was being thus disturbed ? They would thus drag upon the 

 bottom, like the floats of the paddle-wheels, and so set up an 

 accumulative action. 



The opposing action of the off-shore gale would, of course, in 

 time, altogether check and reverse the motion of the particles, 

 and propagate waves in the opposite direction. 



The composition of a beach, no doubt, affects the question to 

 some extent. It seems obvious that where a beach is composed 



