THE LAW OF RESISTANCE IN PARALLEL CHANNELS. 
941 
The queries, as they are put, suggest two methods of experimenting :— 
(1.) Measuring the resistances and velocities of different diameters, and with 
different temperatures of water. 
(2.) Visual observation as to the appearance of eddies during the flow of water 
along tubes or open channels. 
Both these methods have been adopted, but, as the questions relating to eddies 
had been the least studied, the second method was the first adopted. 
9. Experiments by visual observation. —The most important of these experiments 
related to water moving in one direction along glass tubes. Besides this, however, 
experiments on fluids flowing in opposite directions in the same tube were made, also 
a third class of experiments, which related to motion in a flat channel of indefinite 
breadth. 
These last-mentioned experiments resulted from an incidental observation during 
some experiments made in 1876 as to the effect of oil to prevent wind waves. As 
the result of this observation had no small influence in directing the course of this 
investigation, it may be well to describe it first. 
10. Eddies caused by the 'wind beneath the oiled surface of water. —A few drops of 
oil on the windward side of a pond during a stiff breeze, having spread over the pond 
and completely calmed the surface as regards waves, the sheet of oil, if it may be so 
called, was observed to drift before the wind, and it was then particularly noticed that 
while close to, and for a considerable distance from the windward edge, the surface 
presented the appearance of plate glass ; further from the edge the surface presented 
that irregular wavering appearance which has already been likened to that of sheet 
glass, which appearance was at the time noted as showing the existence of eddies 
beneath the surface. 
Subsequent observation confirmed this first view. At a sufficient distance from the 
windward edge of an oil-calmed surface there are always eddies beneath the surface 
even when the wind is light. But the distance from the edge increases rapidly as the 
force of the wind diminishes, so that at a limited distance (10 or 20 feet) the eddies 
will come and go with the wind. 
Without oil I was unable to perceive any indication of eddies. At first I thought 
that the waves might prevent their appearance even if they were there, but by careful 
observation I convinced myself that they were not there. It is not necessary to 
discuss these results here, although, as will appear, they have a very important 
bearing on the cause of instability. 
11. Experiments by means of colour bands in glass tubes. —These were undertaken 
early in 1880 ; the final experiments were made on three tubes. Nos. 1, 2, and 3. The 
diameters of these were nearly 1 inch, \ inch, and ^ inch. They were all about 4 feet 
6 inches long, and fitted with trumpet mouthpieces, so that water might enter with¬ 
out disturbance. 
The water was drawn through the tubes out of a large glass tank, in which the 
MDCCCLXX XIII. 6 E 
