148 MR JOHN AITKEN ON 



over 5 litres in one case and a little under 5 in another, which shows that about 

 ^ of sand is air space. The garden soil in the test vessel would probably be as 

 closely packed as the soil in the garden, but the sand would not be so firm as it was in 

 its undisturbed condition, as I found it possible, by continued shaking while in 

 the water, to make it more compact, and bring fully \ a litre of water to the 

 surface. 



These figures seem to indicate that about \ of cultivated soil is air space, and 

 about \ is air space in consolidated sand. With these figures it would be easy to 

 calculate the amount of air that each cubic foot would send to the surface with 

 a given fall of the barometer. But this will not help us much, as it is difficult to 

 get the air space in the lower undisturbed strata and find the mean depth from 

 which the air rises. From the porosity of many soils it is evident that a consider- 

 able amount of air must ascend and descend with every change of pressure, and 

 this air will have properties different from the air in the general circulation, and 

 may give rise to certain physiological effects. 



PART II. 



(Read May 7, 1900.) 



In the experiments described in the first part of this paper, the cyclonic move- 

 ments were produced under very artificial conditions, the centre of low pressure 

 being kept in a fixed position and the ascending column of air protected by solid 

 walls. It seemed, however, that if the views set forth in Part I. were correct, that 

 there was no reason why these experimental illustrations might not be extended, 

 and made without much apparatus, and in free air, so as to allow us to study the 

 motion of the centre of the depression as well as the spiral movements of the air 

 towards it. On trial I found this could be easily done, and soon had the pleasure of 

 seeing small cyclones forming and travelling across the experimental area, and the 

 spirally inflowing air was seen moving towards the onward moving centre of low 

 pressure. 



The apparatus required for these experiments is very simple, and consists of a 

 small platform, the surface of which can be heated for supplying the hot air required 

 to make the cyclones. On this hot surface a wet piece of cloth or paper may be 

 laid, or fumes may be formed, as in the previous experiments, to enable the eye 

 to follow the movements of the air. Over this heated area, under certain conditions, 

 cyclones are seen to form and travel across its surface. To enable the experimental 

 surface to be heated, it was made in the form of a shallow tin box, closed on all 

 sides and provided with two small pipes, one for the entrance of steam to heat it, 

 and another for draining away the condensed water. This box is 75 cm. square 



