1915-16.] The Dynamics of Cyclones and Anticyclones. 179 
full of water. If before opening the outlet we allow the water to come to 
rest, then in passing out it flows in radial lines towards the outlet, converg- 
ing from all directions towards the opening, but without any cyclonic 
motion being formed. If now, instead of allowing the water to come to 
rest before opening the outlet, we give it a slight circular motion, the flow 
of the water towards the opening is entirely changed. The water is not 
drawn from the bottom, but from a small area over the outlet, and soon the 
whole core is sucked out of the centre of the circling water, leaving the 
well-known hollow space. 
One naturally asks, Why this difference ? Why does the water in 
one case run out from all directions, while in the other it tends to be 
drawn from the upper layers ? A little consideration of what is taking 
place in the basin will enable us to understand the reason for the differ- 
ence in the two cases. When the water is free from motion the whole of 
the moving force acts radially from the outlet and the water flows direct to 
the opening. But if the water is rotating, this radial force has to contend 
against the centrifugal force of the water, which centrifugal force opposes 
the flow towards the centre ; and this opposition is increased by the suction 
drawing the water towards the centre, so augmenting its angular velocity 
and its centrifugal action. The result of this is that the circular movement 
of the water, increased by the inward suction^ surrounds the outlet with a 
resisting tube of circling water. But while the sides of this tube offer 
resistance to the entering water, the top of the tube is, so to speak, 
open. The horizontally circling water offers no resistance to the vertical 
pressure, so the water flows in at the top ; but in the act of flowing in, 
its centrifugal force is increased, with the result that the resisting 
tube of water is lengthened, and this lengthening process goes on till 
the resisting water tube reaches the surface, after which the indraught 
increases the centrifugal force to such an extent that it is able to hold 
back the weight of the surrounding water — the depth of the hollow 
depending on the amount of suction and the initial rate of revolution 
given to the water. 
To elucidate this point a simple experiment may be made which is 
illustrated in the accompanying sketch. Fig. 1, A, is a cylindrical glass 
vessel; a large-sized beaker, 20 by 10 cm., does very well for the 
purpose. The beaker is filled with water, into which is stirred some saw- 
dust made from a heavy wood, to show the movements in the water. B is 
a siphon about 5 mm. in diameter, by means of which the water is 
emptied out — from the top, in this case ; the tube being lowered as the 
level of the water falls. If the water be free from motion before the 
