116 



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



Other experimental researches on the flow of the Mississippi River, 

 much more elaborate than those of Ellet, were made in the period 

 between 1850 and 1861 by Captain Humphreys and Lieutenant Abbot, 

 with others acting under authority from the American Government, 

 and an account of them was published as a Report by Humphreys 

 and Abbot in 1861.* These experiments and the investigations exhi- 

 bited in the report, where the observed results are combined in various 

 ways so as to bring out average results and more or less probable con- 

 clusions for various circumstances, lead very clearly and very con- 

 vincingly to the conclusion that ordinarily the maximum velocity is 

 not at the surface but at some depth below it, usually much nearer to 

 the surface than to the bottom, and often at some such depth from the 

 surface as ^ or -J of the whole depth of the water. These investigators 

 (Humphreys and Abbot) show further (at pages 285, 288, and 289 of 

 their Report) that this phenomenon is not wholly nor even mainly due 

 to any frictional resistance applied by the superincumbent atmosphere 

 to the forward flow of the surface of the water ; because they found 

 that even when the wind is blowing in the direction of the river 

 current, and advancing at the same velocity as that current, so that 

 the air lies on the surface of the water without relative motion, the 

 phenomenon manifests itself almost in as great a degree as when the 

 air is lying at rest relatively to the land ; and found yet further that 

 the phenomenon still manifests itself even when the wind is blowing 

 in the direction of the flow of the river much faster than the current, 

 so that it blows the water surface forward instead of applying a 

 resisting drag or backward force to the surface. 



At about the middle of the present century very important experi- 

 ments on flowing water were made in France by Boileau, and by 

 Darcy and Bazin ; and elaborate accounts of these researches were 

 published.f 



The experiments comprised among the researches of Boileau and of 

 Darcy and Bazin, to which I have to refer as bearing on the special 

 subject of the present paper, relate to the flow of water in long channels 

 and conduits constructed artificially, some in wood and some in ma- 

 sonry and other materials. The channels or conduits in different 

 cases were of widths comprised between half a metre and two metres. 

 In some of the more important experiments the channels were con- 



* Report on the "Physics and Hydraulics of the Mississippi River." By Captain 

 A. A. Humphreys and Lieutenant H. L. Abbot. Philadelphia : 1861. 



f Boileau: " Traite de la mesure des eaux courantes." Paris : 1854. Darcy :" Re- 

 cherches expermientales relatives au mouvement de l'eau dans les tuyaux." Pai*is : 1857. 

 Darcy et Bazin : " Recherches Hydrauliques." Paris : 1865. This last book con- 

 stitutes a memoir by Bazin on researches commenced by Darcy, and continued for 

 some time by him with the aid of Bazin ; and, after the death of Darcy in 1858, 

 continued by Bazin, and by him completed and worked out in the discussion of their 

 results. 



