136 MR JOHN AITKEN ON 



If there is no cyclonic motion formed when the gas is lit, any light objects lying 

 on the table are not disturbed by the radially moving air, but if a good cyclonic 

 circulation is set up, then any light bodies, such as sawdust, paper, or tufts of cotton 

 wool, which happen to be lying under the chimney, are seen to be lifted up and tossed 

 about, generally getting thrown out of the centre of the cyclone by their centrifugal 

 force, but the cotton wool is frequently drawn up the centre and discharged at the 

 top of the tube. Should there be no current in the room suitable for producing the 

 tangential motion, then the cyclone may be formed by blowing by artificial means 

 across one side of the area, or over two or more sides if in correct directions. 



Let us now study more closely the movements of the air under the draught tube. 

 Suppose we have it arranged so that a gentle current is blowing across one half of 



the cyclonic area, and the air is forming a cyclone under 

 the tube, we shall now place the small vanes — already 

 referred to — within the area of the cyclone, to show the 

 directions of the movements of the air — that is, the winds 

 at the different parts of the cyclone. The result is roughly 

 shown in fig. 2. The air current over the table is supposed 

 to be blowing in the direction of the arrow at the side, and 

 to be strongest on that side of the area, and the cyclonic 

 movement is roughly indicated by the arrows in the 

 diagram. It is impossible to get these directions satis- 

 factorily indicated by the vanes, owing to the difficulty of 

 Fig. 2. keeping the cross current constant, and the amount of 



curving of the spirally-moving air is constantly changing 

 with every change in this tangential component. The arrows, however, show that on 

 the side where the tangential current is strongest the in-draught is most tangential,, 

 whilst on the opposite side the incoming air moves more radially, or, to put it more 

 generally, the air in the right-hand front of cyclones in our area moves more tangentially 

 than the opposite left-hand rear position. It should be noticed here that these remarks 

 apply to the case where the centre of the cyclone is fixed. Some modification will be 

 necessary when applying them to cyclones in which the centre of low pressure is in 

 motion. 



We may learn something further with this apparatus if we use fumes to study 

 the movements of the air over the different sections of the cyclonic area. One very 

 marked result which will be observed is that the air — as shown by the fumes — near 

 the surface of the table at all parts of the area moves much more radially than the air 

 higher up, and also that the air lying on the surface of the table is drawn into the very 

 core of the cyclone, up which it rises in a rapidly-circling path of small diameter, whilst 

 the air higher up comes towards the centre along a rising wider spiral path, and forms 

 the outer lining of the cyclonic tube. The lower air keeps near the surface till it arrives 

 near the centre of the cyclone and then rises, making many more revolutions than the 



