DYNAMICS OF CYCLONES AND ANTICYCLONES. 147 



circumference by the cyclone and drawn into the cyclone curving round the front of 

 it, and in its passage it gets added to it all the hot moist air in front of the cyclonic 

 area. After the cyclone is past it is evident that, as it has drawn up the surface air 

 over the area of its track, that what was previously the higher, drier, and purer air 

 will have taken its place ; hence the coolness, freshness, and purity of the air after the 

 disturbance has passed. The cyclone has brought down to us the air we would have 

 got if, before it passed, we had ascended a considerable distance from the earth's surface. 



The above seems to give a fair explanation of the presence of the hot moist 

 air in front of cyclones. Still, some may feel inclined to say that is not the whole 

 explanation, and that there is something else in the air over that area which gives 

 it a peculiar heavy feel — something, in fact, to which no hygroscope is sensitive. To 

 explain these peculiar sensations produced by this air, there is a point to which 

 reference might be made. It is one which I do not think has ever been investigated 

 or even referred to, and it is one which may help to explain the peculiar physiological 

 effects experienced under these conditions. In front of a cyclone where the pressure 

 is falling there will be a considerable amount of air rising from the soil and rocks 

 underneath the surface. This air will come charged with moisture, and more or less 

 changed in its composition by contact with the soil ; it will also bring with it any 

 impure gases there may be in the soil. That such air does rise there can be no 

 doubt. If we observe the surface of the ground while the barometer is falling, 

 after a slight fall of snow which has come before the frost, so that the temperature of 

 the ground is not below the freezing point, we shall see little bare spots where the 

 snow has melted. As these little patches are scattered all over bare ground, gravel 

 walks, etc., where the subsoil is uniform, they can hardly be due to heat conducted 

 upwards, but rather seem to be the result of hot air rising from the ground. The fact 

 that the ground under these bare patches is more porous than elsewhere also points 

 to the rising air as the cause of the melting. Though moist air rises from under 

 the soil with a falling barometer, yet it is difficult to get any satisfactory way of 

 measuring its amount. We can easily find out the air space in any sample of 

 soil, and a few tests have been made in this direction. In place of taking the air 

 capacity, the unoccupied space was measured by means of water. A cylindrical 

 vessel that held 17 litres of water was used. In making the tests this vessel was 

 filled with the soil, firmly tramping in layer by layer ; water was then added till 

 it showed on the top. In making this measurement it is necessary to fill the vessel 

 with water from below, otherwise it will not penetrate all through the soil. In the 

 tests this was done by thrusting a small pipe down through the centre of the soil, and 

 pouring in the water at the top of the pipe. Well-packed garden soil, tested in 

 winter, when it was wet, gave the following result. In one case 3 litres of water 

 were required to fill up the air spaces, and in another it required 3*4 litres, so that 

 something between i to ^ of garden soil is air space. A clay soil would probably 

 have less air space. When sand was treated in the same way, it required a little 



VOL. XL. PART I. (NO. 7). X 



