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



that stage of great relative velocity, it would at an earlier stage 

 ruffle up the mutual face of meeting of itself and the water into pro- 

 tuberances and hollows, somewhat like waves, on the principle referred 

 to already in a foot-note as having been proposed by Sir William 

 Thomson, and would carry this action on to the extent of causing 

 commotion and commingling of the water and oil. The contrast be- 

 tween this case and that of an ordinary river of water is so remarkable 

 as to aid the forming of a clear comprehension of the very different 

 mode of action which I have been attributing to the water in ordi- 

 nary rivers and other open channels. 



It is further worthy of notice that if, from any local cause, the water 

 flowing forward in some part of the width of a river has in its motion 

 a component downward from the surface towards the bottom, and is 

 free from intrusion of upward currents or rushes of deadened water 



Academy of Sciences on the memoir of M. Bazin, " Sur le Mouvement de l'Eau dans 

 les Canaux decouverts." In that report the committee remark (as confirmatory of 

 the view which they accept, to the effect that in deep rivers, especially when not very 

 wide relatively to their depth, the place of maximum velocity is at a considerable 

 depth below the surf ace) as follows : — " II y a longtemps que les bateliers du Rhin et 

 nos pontonniers savent qu'iin bateau charge et ayant un fort tirant d'eau, marche, en 

 descendant, plus vite que 1'eau qui le soutient ou que les corps flottantsala surface." 

 This obviously conveys the opinion that a heavily loaded boat, sinking deep into the 

 water, and thereby having its deeper part immersed in water which is flowing quicker 

 than the surface water, is dragged forwards by that deeper and quicker moving- 

 water, and so is made to advance quicker than the surface water does. The idea 

 seems to be that the boat has some average velocity less than that of the water at its 

 bottom, and greater than that of the surface water. The view which thus appears 

 to be held in respect to the observed phenomenon seems to me to be inadequate and 

 erroneous. On the principle put forward above in the present paper in reference to 

 the imagined case of a river with an upper layer of oil, I would suppose that a large 

 and heavy boat, even if flat-bottomed and of shallow draught of water, would run 

 down the river-course quicker than the water in which it swims ; for the reason that 

 while all the water surrounding it makes occasional visits to the bottom of the river, 

 and meets with great retardation there, the boat does not dive to the bottom, and is 

 free from any such retardation, and so is only held back by the surrounding water 

 against taking from gravity a perpetually increasing velocity. Thus it must go faster 

 than the surrounding water wmich has to hold it back. The boat of deeper draught 

 referred to by the committee I would suppose would advance quicker than the surf ace 

 water, for the same reason, and not merely because of its bottom being situated in 

 water moving quicker than that at the surface. The principles I have assigned would 

 afford ample reason for our supposing that the boat of deep draught might swim 

 forward much quicker not only than the surface water, but also than the water at its 

 bottom, or indeed than any part of the water of the river surrounding the boat. 

 Yery small floating objects, such as sticks or leaves, would present, in proportion to 

 their small masses, so much resistance to motion through the surrounding water that 

 they would be constrained in fact to move sensibly at the same velocity as that of the 

 water surrounding them. The phenomenon would thus be presented of the boat 

 swimming forward past the small floating objects around it. 



J. T. 



