1 18 Prof. J. Thomson on the Flow of Water in [Dec. 12, 



to me to contain the nearest approach towards an explanation of the 

 phenomenon in question of any that have been attempted, so far as 

 any such attempts have come under my notice. It involves, I think, 

 at least a glimmering towards a true explanation ; but I regard it as 

 being in great part erroneous, and importantly so in principle, and as 

 being besides altogether incomplete. I do not think it has been offered 

 by the very able investigator himself, who has proposed it, as being at 

 all sufficient ; but I think it has been offered only as tending to throw 

 some light over the region for further search, and some indication to- 

 wards courses in which speculation and research might well advance. 



Bazin's experiments, of the general character already mentioned, 

 were very extensive in their scope, and were carried out in great detail, 

 and with some remarkable refinements of method. The velocities 

 were measured mainly or wholly by a modification devised by Darcy 

 of the well known instrument called Pitot's tube. Bazin, in the case 

 of canals not very wide relatively to the depth of the current, found 

 very clearly and decisively the phenomenon in question of the maxi- 

 mum velocity being below the surface. But, in the case of rect- 

 angular channels of more considerable width, channels having the 

 width of the current so much as four or five times the depth or more, 

 Bazin by his scrutiny and consideration of his experimental results, 

 was led to conclude that the diminution of velocity for approach 

 towards the surface in the upper part of the current is to be found 

 only in the side parts of the current — the parts flowing along the two 

 side walls. He judged that throughout the whole of the current, 

 except two side parts, each having some moderate width, which might 

 be equal to about twice the depth of the current, the maximum of the 

 velocities for all points, situated in a vertical line, is to be found at the 

 surface ; and that the rate of diminution of velocity for descent from 

 the surface would begin as nothing at the surface, and would go on 

 increasing with descent to the bottom. His experiments, according to 

 his own careful analysis and combination of them, appeared to be in 

 agreement with this assumption, or to bring this supposition out as a 

 result. 



I do not, however, regard this conclusion as being trustworthy. His 

 experiments for the case of great width relatively to depth had not, in 

 any instance, a depth of water exceeding *38 of a metre, or 1\ foot, 

 and thus the depths were so small absolutely as not to admit of a fine 

 enough discrimination of minute changes of velocity for minute 

 changes of depth of the point where the velocity was observed, nor 

 of measuring velocities close enough to the surface. So far as experi- 

 mental researches go, some doubt I presume must still remain over 

 this part of the subject. Indeed, the Indian experiments, next to be 

 mentioned, show results in disagreement with this conclusion offered 

 by Darcy. 



