190 
HINTON. 
let C D and E F be two rods crossing them. Now, in the 
space of x y z if the rods turn round the lines A and B in 
the same direction they will make 
two independent circles. 
When the end F is going down 
the end C will be coming up. 
They will meet and conflict. 
But if we rotate the rods about 
the plane of A B by the z to w 
rotation these movements will 
not conflict. Suppose all the fig¬ 
ure removed with the exception 
of the plane x z, and from this 
plane draw the axis of w , so that we are looking at the space 
of x z w. 
Here, Fig. 10, we cannot see the lines A and B. We see the 
points G and H, in which A and B intercept the x axis, but 
we cannot see the lines themselves, for they run in the y di¬ 
rection, and that is not in our drawing. 
Now, if the rods move with the z 
to w rotation they will turn in paral¬ 
lel planes, keeping their relative po¬ 
sitions. The point D, for instance, 
will describe a circle. At one time 
it will be above the line A, at another 
time below it. Hence it rotates 
round A. 
Not only two rods, but any num¬ 
ber of rods crossing the plane will 
move round it harmoniously. We can think of this rotation 
by supposing the rods standing up from one line to move 
round that line and remembering that it is not inconsistent 
with this rotation for the rods standing up along another line 
also to move round it, the relative positions of all the rods 
being preserved. Now, if the rods are thick together, they 
may represent a disk of matter, and we see that a disk of 
matter can rotate round a central plane. 
