DYNAMICS OF CYCLONES AND ANTICYCLONES. 135 



below water. By these means air was prevented entering either end of the discharge 

 pipe. 



For making experiment on the cyclonic movements in air, I have found the 

 following piece of apparatus useful. To produce the up-draught a thin metal tube 

 15 cm. diameter and fully 2 m. high was used. At the lower end of the tube is 

 fixed a circular disc 76 cm. diameter. The disc is supported on three legs, 

 15 cm. high, thus leaving an air space of 15 cm. between the disc and the table 

 on which it rests. To produce the up-draught three small jets of gas are fitted 

 inside the tube near the lower end. To study the circulation produced by the 

 apparatus, a number of small light vanes supported on small stands were used, so 

 that they could be put to show the direction of the air currents anywhere within the 

 area affected. These vanes show the directions of the circulation at the different parts 

 of the area affected by the cyclone, but for studying the variations in the circulation 

 at different heights from the surface of the table, the fumes from hydrochloric acid 

 and ammonia were found to be more useful. These fumes are very suitable for the 

 purpose, as they have only a very slight proper motion of their own, rising only 

 very slowly, so that their own movements do not interfere much with the cyclonic 

 motions. When working with fumes a large plate of glass should be placed below 

 the apparatus to give freedom for experimenting, and the fumes are best made by 

 placing pieces of paper on the glass and dropping the ammonia and acid on them. 

 These papers, being close to the surface, do not give rise to local eddies. 



Before lighting the gas to make an experiment, some fumes should be made 

 under the tube, to see if there are any air currents in the room. Suppose there are 

 none, and the fumes rise slowly without drifting in any direction, the gas should 

 now be lighted and a number of centres of fumes should be started at different 

 points under and round the tube, either by dropping the acid and ammonia on 

 separate pieces of paper, or by putting them in watch glasses. If we observe the 

 fumes under these conditions, it will be seen that they rise and move radially towards 

 the hot chimney, moving upwards in even curves, but showing no tendency to rotate 

 round the centre. An element is still wanting to produce the cyclonic motion, the 

 air must be given some initial movement before the up-draught will take the spiral 

 form. Suppose that on first testing the air on the table, we found that it was not free 

 from movement, but that there was a slight current blowing across it. If this current 

 be equally strong at all points, it will not be of much use in generating cyclonic move- 

 ment, but if we put up a screen so as to cut off the current from one side of the area 

 and allow it to blow on the other, then we have the conditions necessary for producing 

 vortex motion under the chimney. If we examine the fumes rising under these new 

 conditions, it will be seen they no longer move radially but are in violent cyclonic 

 motion, swirling round and round in the direction given by the tangential current, the 

 rising fumes forming graceful ascending spirals. So strong is the circular motion that 

 at times the gas in the chimney is heard flaring as in a strong wind. 



