122 Prof. J. Thomson on the Flow of Water in [Dec. 12, 



water surface, and they may be seen presenting appearances of spread- 

 ing out till they meet one another and give indication of momentary 

 downward sinking at their places of meeting. 



From whence do these transverse currents come to the surface ? It 

 seems to me they must have had their origin in the deadened water 

 scouring along the bottom, or along the wetted side-faces of the channel, 

 in such ways as have just now been briefly sketched out. Thus it seems 

 that there are tendencies bringing about the result that the superficial 

 stratum of the river receives perpetually renewals of its substance by 

 water currents arriving to it, and spreading out there, which have very 

 recently departed from the bottom before coming up to enter into that 

 superficial stratum. But their substance, having come in great part 

 from the bottom, must be largely made up of the deadened or slow- 

 going bottom-water. It is to be understood that this deadened water, 

 in rising through the current towards the surface, is partly urged for- 

 ward in the down-stream direction by the surrounding quicker-going 

 water, but that it arrives at the surface without having attained fully 

 to the down-stream velocity of that intermediate stream. 



It may readily be perceived that it is from the washed face of the 

 channel alone, or from that and the retarded layer of water in proxi- 

 mity to it, that any strong transverse impulses can be applied to any 

 parts of the current. No rapid transverse current will originate in the 

 middle of the body of the river ; for there is no cause for the origination 

 of transverse currents there, unless perhaps we were to regard as such 

 any slight transverse motions which may be produced through the 

 gliding forward of parts of the water there relatively to others near 

 them going with different velocities, and unless we were to regard as 

 such any transverse disturbances that may be imparted to forward- 

 flowing water there by the intrusion and commingling of partially 

 deadened water from the channel-face. 



We may now have great confidence, I think, in taking as a well- 

 established truth, or at least as a very probable view, the supposition 

 already laid down to the effect that very commonly the superficial 

 stratum of a river receives perpetually renewals of its substance by 

 water currents arriving to it and spreading out there, which have very 

 recently departed from the bottom or sides of the channel before 

 coming up to enter into that superficial stratum ; and that the substance 

 thus perpetually renewing the surface stratum is largely composed of 

 deadened or slow-going bottom- water, or of water going slower forward 

 than the water through which it traverses in ascending to the surface. 

 It is further to be noticed that the water which at any moment consti- 

 tutes the superficial stratum is, in its turn, very soon overflowed by 

 later arrivals from the bottom. So it gradually descends from the sur- 

 face into the interior of the body of the river. But during this action 

 it is always flowing downhill, or we may better say it is experiencing 



