132 MR JOHN AITKEN ON 



air be at rest relatively to the centre of low pressure — that is, that the centre of low 

 pressure and the air surrounding it are all moving in the same direction and at the 

 same velocity. Another qualification necessary in the case of the air on the earth's 

 surface is, that the area of low pressure be very small, otherwise the different parts of the 

 area will have different rates of movement owing to the earth's rotation causing its 

 surface and the air near it to move faster near the equator than near the poles, so that 

 the air to the south of the low-pressure centre moves more quickly in an easterly direc- 

 tion than the air to the north of it — that is, supposing there is no wind. 



Let us now return to the question of the cause of the cyclonic movement. That 

 motion relatively to the low-pressure centre is necessary to produce the spiral approach 

 of the air or water to the centre is easily illustrated. Take a circular vessel — say 

 40 cm. in diameter and 15 cm. deep — filled with water. The vessel should have an 

 opening in the centre which can be closed from the outside, so that it can be opened 

 without disturbing the water in the vessel. Put some sawdust, or similar substance, in 

 the water and mix it with it. Now leave it at rest till all movement in the water 

 ceases, then open the outlet in the bottom of the vessel and allow the water to run 

 away, when it will be seen that the sawdust suspended in the water moves towards the 

 outlet flowing from all directions, and at all depths, in radial lines, without a trace 

 of vortex movement. 



Let the conditions be now changed and a slight circular motion be given to the 

 water before the outlet is opened. A well-formed cyclone will now be formed by the out- 

 flowing water, and the quicker the initial movement the greater will be the number of 

 turnings in the spiral path of the water before it arrives at the outlet, and the deeper 

 the hollow in the surface of the water over the outlet, while the direction of the spiral 

 movement will be the same as the initial movement given to the water. The depression 

 of the surface of the water at the centre of the vortex is extremely interesting. If the 

 water before the outlet be opened be not perfectly at rest, a faint depression will always 

 be detected over the outlet, having in its centre a quickly-rotating vortex of very small 

 area. If, however, the initial motion be greater — but it need not be quick — then the 

 depression deepens and forms an air-tube extending to the bottom of the vessel, showing 

 that the slow initial circular motion has enabled the low-pressure area to generate a 

 velocity at the centre of the cyclone sufficient to enable the centrifugal force of 

 the water at that part to withstand the hydrostatic pressure at the bottom of the 

 vessel. 



On examining these water vortexes there is a point that strikes one as very remark- 

 able — namely, the great increase in the velocity of the water as it approaches the centre 

 of movement. Not only is the angular velocity greatly increased owing to the shorter 

 path required to complete a revolution near the centre, but the absolute velocity is also 

 greatly increased. Perhaps this point can be more easily seen by making the experi- 

 ment in another form, and using solids in place of liquids. Suspend two balls, A A, 

 fig. 1, by long fine wires, either from the same point of suspension, or, to reduce the 



