Mr. A. R. Hunt. 



[Apr. 20, 



nate currents. I found on trial with a small glass aquarium, contain- 

 ing line blown sand, that an alternating current passing over a space 

 of 2 inches 120 times per minute strongly rippled the sand with 

 ripples varying in size from one inch downwards. Thus an average 

 speed of 4 inches per second sufficed to form ripples so large as one 

 inch from ridge to ridge. It is clear that a much slower current would 

 still suffice to produce smaller ripples. 



It would be a matter of great interest to know at what depth an 

 Atlantic wave would set up an alternating current of 4 inches per 

 second, for, whatever the depth, it would fall short of that at which 

 ripplemarks might be formed. 



Although the question of the relation of current action to depth of 

 water, and to height and length of wave, is one that must be left to 

 the mathematician, and is one with which I cannot pretend to grapple, 

 the following experiments, though on a small scale, may be worth 

 recording : — 



Dried peas placed on a glass plate in a slight depression on a sandy 

 bottom in 6-inch water were rolled off by waves about 12 inches long, 

 and about 1 inch high. Although the motion was due to the waves, 

 the fact that the peas were ultimately rolled off the plate was due to 

 the difficulty of getting the glass perfectly level under water. Shorter 

 waves 1-J inches high had much less effect on them. A little sand 

 that had collected on the glass was beautifully rippled with J inch 

 ripples ; these were dried and varnished. As it was difficult to discern 

 slight motion of any object owing to the undulation of the water, I 

 proceeded to make a rough indicator, whereby a vane of thin wood 

 placed at the bottom at right angles to the course of the waves would 

 communicate a multiplied motion to a long light needle above the 

 surface. 



With this rough machine I tried the following experiments : — 



(1.) Water, 8 inches; to top of vane, 7 inches. Waves, 90 per 

 minute; motion by indicator very marked. 



(2.) Waves, 115 per minute, half inch high ; motion doubtful. 



(3.) Waves, 86 per minute, 1 inch high; motion at bottom very 

 strong ; index striking both checks. 



(4.) Waves, 80 to 90 per minute f of an inch high; motion at 

 bottom strong. 



This last experiment was an unintentional one. The larger waves 

 had made a strand for themselves, and I found that by reducing the 

 displacement of the wave generator, I could make smaller waves that, 

 without altering the mean level of the water, rippled the lower part 

 of the former strand which they by their smaller reflux failed to 

 uncover. Thus by a slight alteration of adjustment of the generator, 

 I could, by varying the height of the waves, form or destroy ripple- 

 marks at pleasure. Whilst making these experiments, I chanced to 



