150 MR JOHN A1TKEN ON 



starting the cyclonic movement in these experiments is disposed of by the experiment 

 previously described, in which it is shown that cyclones are formed in still air when a 

 horizontal current is made by blowing across one side of the ascending column, and 

 without the intervention of a screen. The screen in these experiments is only used as 

 a convenient method of protecting one side of the hot area from the horizontal current. 

 If we wish to be more realistic we may crush up a piece of paper roughly into the 

 form of a mountain ridge and put it in the place of the screen, with the result that 

 cyclones go on forming as before. 



When the air from the natural draught in the room flows over the hot area at the 

 same velocity at all parts, the rising steam does not ascend far, but keeps close to the 

 hot surface, and is irregular in its movements. But when a cyclone is formed, most of 

 the steam is collected into a rapidly whirling vertical column, which ascends to a 

 considerable height — to a metre or more above the hot surface, and often presenting the 

 appearance of a well-defined column, as it rises through the clearer air. 



In making these experimental cyclones, it was noticed there must be a definite 

 relation between the amount of heating and the velocity of the cross current. If the 

 heating be slight the cross current must be slow, otherwise the cyclonic movement 

 will not be properly formed owing to the weakness of the ascent, before it is swept 

 off the experimental area ; whilst with hotter air a stronger current may be permitted, 

 with the result that a more violent cyclonic movement is produced, which penetrates 

 the upper air to a greater height. 



The use of steam in these experiments is evidently open to many objections, 

 other than the great amount of heat required to evaporate the water. Although it is 

 water that plays a part in these experimental cyclones as well as in those in nature, 

 yet in the two cases the part played by the water is reversed. In nature the water 

 goes up as vapour, and when it condenses it liberates a great amount of heat, which 

 prevents the temperature of the air falling so much as it would if no condensation 

 took place, so aiding in the ascent of the column. But in the experimental cyclones 

 this action is reversed, much of the water goes up in the cloudy or condensed form, 

 and as in its ascent it gets mixed with more air, the water particles get evaporated 

 and heat is absorbed. So that whilst the water taken up as vapour in the cyclones in 

 nature increases the energy of the motion, it checks the movements in the artificial 

 ones. Condensed water vapour or what we often call steam is thus not the best thing 

 to use for showing these cyclonic movements, but it has the advantage that it is pro- 

 duced all over the experimental area, so that we can by means of it see the beginning 

 and trace the movements of the cyclones. Fumes of hydrochloric acid and ammonia 

 may be used, but in that case it is necessary to cover the experimental area with a 

 sheet of glass, which is almost certain to crack with the heat. If the glass plate is 

 not used the acid and ammonia may be put in watch glasses. Working with fumes 

 we get a much greater violence of movement and greater ascent, owing to the air 

 being more highly heated, but as the liquids evaporate very rapidly they require 



