Aprit 28, 1870| 
NATURE 
655 
the greatest velocity was not at the surface, but at some | 
distance below it. 
Supposing that water moves in an innumerable number | 
of circles, varying from a single particle in diameter to 
that of hundreds of feet, and that every obstruction sets 
these circles revolving at right angles to their surfaces, 
we can at once begin to understand how, by increasing 
the areas exposed to friction, an innumerable set of wheels 
of various sizes are set spinning in all directions, but are 
retarded in this action by the attraction of the several par- 
ticles to each other. Thus wheels within wheels will be set 
in motion, some revolving in opposite directions ; and the | 
quicker the revolutions—that is, the smaller the diameter | 
of the wheels, in other words the shallower the stream— | 
the greater will be the power expendend, which power | 
Nature exerts in holding solid matter in suspension ; 
therefore, if the foregoing arguments be correct, it is | 
evident that the transporting and abrading power of | 
water must increase in some ratio inversely as the depth, 
and that the retarding of a ship’s sailing on a flowing 
river must depend on the increased area of surface 
exposed, thus explaining why a ship with a foul bottom, 
a rough, rocky bed toa river, or weeds in a stream, all 
retard velocity, because they one and all set so many more 
wheels spinning. This leads us to the important questions 
where abrasion and the power of flowing water to hold solid 
matter in suspension have to be investigated, with the 
view of showing how this rotatory motion acts in nature. 
To do so the following diagram will perhaps give a slight 
idea of the complicated nature of this rotation, the circles 
being supposed to increase in diameter with the depth. 
This diagram is only intended to show the relative motion 
of one set of particles with respect to its neighbouring set 
of particles, each for its own depth of 1, 2, 4, 8, or 16 feet 
deep. Thus where the depth is 16 feet, there would bea 
series of circles 16 feet in diameter rolling within each 
other, where the depth was 8 feet, there would be circles 
of 8 feet in diameter, and soon. That is, with the same 
velocity, the rotation would decrease as the diameters be- 
came greater. 
The various angles with the horizon are re 
| resented by 
| the lines D E, D’ E’, and D” E” h : 
, which show the necessary 
| Slopes, in order that the centre of gravity of each circle 
_ should be equally beyond the point of support A’, and that 
consequently A B, A’ B’, A” BY”, should be all equal ; they 
indicate that where the slope of the surface of the water 
remains in each case the same (say, for example, one foot 
in a mile), the velocity probably increases proportionally 
to the increased hydraulic mean depth, or that where 
the velocities are the same, and the depths differ, the slope 
requires also to vary. Let, for example, the velocity be 
in each case about 5 miles an hour, or some 7s ft. a 
second, while the depths are 5ft., 8ft., roft., and goft. respec- 
tively, the slopes vary from 25 feet in the mile to only some 
4 inches, while the load of solid matter held in suspension 
is about 7 per cent., 5 per cent., 3 per cent., and only yrho of 
the weight of water in each of the above cases respectively. 
With the assistance of the diagram, therefore, it will 
tip. 
at once be seen how the whirling motion given to a stream 
must increase as the depth decreases, and how, by the 
increased agitation, the water is able to hold propor- 
tionally more solid matter in suspension, while the action 
on the bed of the channel must at the same time be 
increased. 
To carry this action to extreme cases it appears evident 
that where the velocities are considerable, and the depths 
only a foot or two, the slopes must become almost precipi- 
tous, while the stream must become semi-fluid mud. or 
transport a large proportion of boulders, and even rocks ; 
in doing which a certain amount of power must be ex- 
pended, and in transporting this solid matter this loss of 
power cannot but retard the flow of the stream. On the 
other hand, it may be assumed that, even with consider- 
able velocities, which at small depths would tear up and 
hurl forward rocks, boulders, sand, and mud, with exces- 
sive depths the water may flow on in almost a compara- 
tively pure state, and instead of holding in suspension 
stones and coarse sand, can only transport fine particles 
of mud. 
T. LOGIN. 
