134 MR JOHN A1TKEN ON 



which might be called a secondary result, comes in and assists the centrifugal force in 

 offering resistance to the change. There are plenty of similar results in physical 

 phenomena, where these secondary results assist in resisting the change, as for 

 instance, when we compress air. Suppose, as in the above case, we reduce the 

 volume to one-half, then we have doubled the pressure, which seems fair enough. 

 But the air, with almost human-like dislike to compulsion, heats in the process, and by 

 so doing helps to resist the compression till it has had time to cool. These remarks 

 may suggest something as to the great power of vortex movements for storing energy. 



The cause of the great increase in the velocity in the balls when they are drawn 

 towards the centre is due to work having been done on them. When the balls are 

 revolving round their common centre, held by the cord at a fixed distance, no energy 

 is communicated to them by the tightened cord, because the force acts at right angles 

 to the direction of movement. But when the balls move in a spiral path, then only 

 one component of the radial force acts at right angles to the direction of motion, whilst 

 another is in the direction of the movement, thus tending to increase its velocity. The 

 same explanation applies to the increased velocity of air or water approaching the 

 centre of cyclones. 



This resistance offered by the spirally-moving air to be drawn into the centre 

 of the cyclone has one important effect. It enables the cyclone to develop more 

 energy than if the air moved inwards radially. When the air moves in radially the 

 supply is so abundant that a great fall of pressure cannot take place, but when the 

 inflow of the air is retarded by the centrifugal force, a greater fall of pressure results, 

 and the energy of the cyclone is increased ; and in addition to this we shall see 

 later that it adds greatly to the efficiency of cyclones considered as circulating 

 engines. 



It may perhaps be as well that I give my reasons for saying that whilst the 

 vortex motion gives rise to strong currents— that is, increase of velocity — it yet 

 retards the inflow, and by so doing it will allow a lower pressure to be formed in 

 the centre. This point can be most easiiy illustrated by means of water vortexes. 

 The circular vessel already referred to was used, the outlet being a short pipe 

 fixed in the centre of the vessel. In each experiment the vessel was filled with 

 water to the same depth, and the time observed that the water took to empty 

 when it had no motion before starting to empty — that is, emptied out without vortex 

 motion — and when a slight movement was given so as to cause it to form a cyclone 

 when emptying. The result was the water took a half longer in the latter case 

 than in the former, showing a great retardation due to the cyclonic motion, and this 

 retardation results in a lowering of the pressure at the centre, and it also prevents the 

 low-pressure area being so rapidly filled up. Care was of course taken in the above 

 experiments that the pipe carrying away the water was always full in both cases, so that 

 the head of water should be the same in both. This was done by placing a glass disc 

 over the outlet and at some distance above it, and making the outlet pipe discharge 



