SEPTEMBER 7, 1899] 
waves, whereas the narrowing of the bands at the sides indicates 
an increase of velocity and reduction of pressure, and accounts 
for the depression of water level, with which you are doubtless 
familiar, at the corresponding part of a ship. 
I will now take amore striking case. If, instead of a circular 
body, we had a flat plate, the turbulent nature of the flow is 
evidently very great, as you will see from the view (Fig. 9), 
which is a photograph of the actual flow under these conditions, 
made visible by very fine air bubbles, and showing water at rest 
in the clear space behind the obstacle. 
We can, however, take steps to reduce this turbulence, and 
you now see on the second screen the flow by means of appar- 
atus which time does not permit me to describe, but which gives 
a slow and steady motion that it would be impossible to improve 
upon in actual conditions of practice, or even, I am inclined to 
think, by any experimental method. Instead of using air to 
make this flow clear, we now allow colour to stream behind the 
plate, and you will see that the water still refuses to flow round 
to the back, and spreads on either side. We have so slow a 
velocity as not to induce vortex motion, but the inertia of the 
particles which strike the flat plate causes them to be deflected 
to either side, exactly as tennis-balls in striking against a wall 
obliquely. The sheet of water is so thick, that is to say, the 
parallel glass plates are so far apart, that they do not enable the 
viscosity of the water to act as a sufficient drag to prevent this 
taking place. 
Mathematicians, however, predicted with absolute certainty 
that with stream-line motion, the water should flow round and 
meet at the back, a state of things that, however slow we make 
the motion in the present case, does not occur owing to the 
effect of inertia. They have drawn with equal confidence the 
lines along which this should take place. Wecould either effect 
NO. 1558, VOL. 60| 
NATURE 
| the line may take. 
449 
this result with the experiment you have just seen, by using a 
much more viscous liquid, such as treacle, or, what comes to the 
same thing, bringing the two sheets of glass nearly close to- 
gether ; and the flow which you are now witnessing (Fig. 10) 
shows the result of doing this. The colour bands in front of the 
plate no longer mix at all with the general body of flow, or are 
unsteady, as was the case in the last experiment, but flow round 
the plate, and flow so steadily, that unless we jerk the flow of 
the colour bands, it is impossible to tell in which direction they 
are actually mcving. It is interesting to note that where the 
divided central colour band re-unites is clearly shown in the 
illustration, 
Whilst I have been dealing with the stream-lines ofa perfect 
liquid, your minds will doubtless have turned to the lines 
along which magnetic and electrical forces appear to act. 
We are possibly further from realising the actual nature of 
these forces, than from a correct conception of the real nature 
of a liquid. Wehave long agreed to abandon the old ideas of 
the electrical and magnetic fluids flowing along these lines, and 
to substitute instead the idea that these lines represent merely 
the directions in which the forces act. Now we can easily see 
that this conception is quite a reasonable one, for in the case of 
the model it is not necessary to have the row of balls actually 
moving in order that the effect may be transmitted along the 
different linesthey occupy. If I attempt to raise the plate upon 
which they rest, the pressure is instantly transmitted through 
the whole row to the top ball along each line, whatever curve 
In the same way, you will remember that it 
was not necessary to have the colour bands actually in motion, 
for, though apparently free to move in any direction, they retain 
| their form for a considerable time, and the path along which 
they would influence each other as soon as the tap is opened 
would be along those lines in which the liquid was flowing before 
it was brought to rest. Hence it is possible, with some suitable 
means, to cause a viscous liquid to reproduce exactly the lines of 
magnetic and electrical induction. In the case of magnetism 
and electricity, it is of course possible, by means of a small mag- 
netic needle or a galvanometer, by exploring the whole surface 
through which magnetic induction or elec ical flow is acting, to 
plot the lines of force for innumerable cases, where we can work 
in air or on the surface of the solid conductor. 
But in this building it seems natural to take as an example 
the case first used by the great man to whom the conception of 
lines of magnetic force is due, for the first reference I have been 
able to find to such lines is in one of Faraday’s earliest papers 
on the indication of electric currents (‘‘ Experimental Researches 
in Electricity,” vol. i. p. 32), in which he says, ‘‘ By magnetic 
curves I mean the lines of magnetic forces, however modified 
by the juxtaposition of poles, which would be depicted by iron 
