in Open Channels or in Pipes or other Ducts. 383 



There was an extensive sandy beach there, lashed by the great 

 waves of the Atlantic Ocean. At a part of that beach a small 

 river or stream flowed to the sea ; but the sandy beach had 

 been thrown up as a bank, at about high-tide level, obstructing 

 what might have been the direct outfall course for the stream 

 into the sea, and causing the stream to turn to its right and 

 to flow eastward for some distance along the back of that 

 sandy bank before finding an opening for flow out to the sea. 

 Thus, at the back of the bank, a little estuary was formed, 

 along which, when the tide was down, the stream w r ould have 

 for a considerable length a nearly level bed ; and into which, 

 when the tide was up, the sea- water entered so as to fill it up 

 to various depths according to the height of the sea-surface. 



I happened to be watching with interest the motions of the 

 water in the little estuary at a time when a considerable depth 

 of water (such as a few feet depth along its mid-channel) was 

 maintained in it by the height of the sea- water outside, and 

 when the slow rising and sinking of the ocean-waves was 

 producing in the estuary a flux and reflux on a small scale 

 like that of the tidal flow in large estuaries*. The motions 

 of the water being indicated by numerous little pieces of sea- 

 weed carried in suspension, I noticed that the water at or very 

 close to the channel-bed reversed its landward or seaward flow 

 always much earlier than did the main body of the water in 

 the channel, less affected by contiguity to the bed. The phe- 

 nomenon being noticed, the reason at once became apparent. 

 The lamina contiguous to the bed, or channel-face, would be 

 always hindered by the frictional resistance of that face from 

 getting into so great a velocity, either seaward or landward, as 

 that which would be attained to by the main body of the water. 

 Then, when the water at the sea-end of the estuary was raised 

 in level, by the arrival of an ocean-wave, so as to give a gravi- 

 tational propulsive influence tending to cause the water to flow 

 landward along the estuary, the main body of the w r ater, in 

 virtue of its inertia with seaward momentum, would continue 

 to flow for some time seaward, flowing as it were uphillf; 

 while the frictionally restrained lamina at the channel-face, 

 being nearly devoid of inertial tendency seaward, would 

 readily yield to the landward gravitational propulsive influ- 



* The period of these oscillations may be about from 10 to 20 seconds ; 

 as I have been informed that Prof. Stokes has found, by observations on 

 that coast, that the period from one wave to the next, in the large 

 Atlantic waves there, is at most about 17 seconds. 



t Or, in more precise terms, flowing from a place of lower to a place of 

 higher free-level. 



