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POPULAR SCIENCE REVIEW. 
motion of the water which formed the ring and of the surround- 
ing water was very clearly seen. It was at once seen that the 
visible ring, whether of coloured water or air, was merely the 
central line of the vortex ; that it was surrounded by a mass 
of moving water, bearing somewhat the same proportion to 
the visible ring as a ball made by wrapping string (in and out) 
round a curtain ring until the aperture was entirely filled up. 
The disc, when it was there, formed the front of this ball or 
spheroid of water, but the rest of the surface of the ball had 
nothing to separate it from the surrounding water but its own 
integrity. Yet when the motion was very steady the surface of 
the ball was definite, and the entire moving mass might be 
rendered visible by colour. The water within the ball was every- 
where gyrating round the central ring, as if the coils of string 
were each spinning round the curtain ring as an axis, the water 
moving forwards through the interior of the ring and backwards 
round the outside, the velocity of gyration gradually diminish- 
ing as the distance from the central ring is increased. 
The way in which the water moves to let the ball pass can 
also be seen, either by streaking the water with colour or sus- 
pending small balls in it. In moving to get out of the way 
and let the ball of water pass, the surrounding water partakes 
as it were of the gyrating motion of the water within the ball , 
the particles moving in a horse-shoe fashion, so that at the 
actual surface of the ball the motion of the water outside is 
identical with that within, and there is no rubbing at the 
surface, and consequently no friction. 
The maintenance of the shape of the moving mass of water 
against the unequal pressure of the surrounding water as it is 
pushed out of the way is what renders the internal gyratory 
motion essential to a mass of fluid moving through a fluid. The 
centrifugal force of this gyratory motion is what balances the 
excess of pressure of the surrounding water in the front and 
rear of the ball, compared with what it is at the sides. 
It is impossible to have a ring in which the gyratory motion 
is great, and the velocity of progression slow. As the one motion 
dies out so does the other ; and any attempt to accelerate the 
velocity of the ring by urging forward the disc, invariably 
destroyed it. 
The striking ease with which the vortex ring, or the disc with 
the vortex ring behind it, moves through the water, naturally 
raised the question as to why a solid should experience resist- 
ance. Could it be that there was something in the particular 
spheroidal shape of these balls of water which allowed them to 
move freely ? To try this, a solid of the same shape as the fluid 
ball was constructed and floated after the same manner as the 
disc. But when this was set in motion, it stopped directly — it 
