DYNAMICS OF CYCLONES AND ANTICYCLONES. 139 



movements could never give rise to anything like a systematized and law-abiding 

 circulation, though they might result in local disturbances such as thunderstorms 

 and rain. 



Let us now suppose the conditions are changed and that an anticyclone is 

 blowing at one side of our supposed hot area, and note the change. When the upper 

 air descends to the earth's surface, it spreads out in all directions, and if the earth had 

 no motion of rotation the air would spread in radial paths from the centre — that is, 

 supposing the descending column of air had no motion of its own, which is very 

 unlikely. But as little is known on this subject, we must at present neglect it, and 

 may safely do so, as probably even if it has motion it will not greatly affect the 

 result. Suppose that the descending air arrives at the surface of the earth and spreads 

 outwards in all directions, then, owing to the earth's rotation, the air in place of 

 moving radially from the centre, goes in spiral paths. This is owing to the air that 

 moves in a northerly direction blowing over a part of the earth that has a slower 

 easterly motion than the descending air which has come from the south, and the wind 

 seems over the northern part of the anticyclone to blow more or less from the west. 

 On the other hand, the air moving southwards of the centre flows over an area going 

 more quickly eastwards than the descending air, and the wind over that part of the 

 anticyclone seems to blow more or less from an easterly direction, whilst the air moving 

 east and west from the centre would appear only to have its velocity altered, were it 

 not for other causes which come into play, and make the circulation from the anticyclone 

 to be, in a general way, spiral all round the centre. As the air to the north of the 

 centre moves eastward, and that to the south westwards, the circulation in the anti- 

 cyclone is right-handed — that is, in the direction of the hands of a watch with its 

 face upwards. 



One effect of spiral circulation, to which we have already referred in the experi- 

 mental part of the paper, is that the air near the surface of the ground where its 

 motion is retarded by friction moves more radially than the upper air. This causes 

 the higher air currents to cross the lower at a greater or less angle, and has the effect 

 of checking the rising of the lower air even when lighter than the upper. In our 

 atmosphere the case is more complicated than in the experiments, owing to the 

 earth's rotation. Over certain areas of the anticyclone the upper and lower airs may 

 be moving in nearly the same direction : this will be the case when the upper air is 

 moving in nearly the same direction as the surface of the earth at the place. But 

 even when the direction of the upper and lower airs is the same, the difference in 

 velocity of the two has some influence in checking the lower air from rising. 



Let us now see what the effect of such an organized circulation is in our 

 atmosphere. Let the sketch fig. 5 represent an imaginary anticyclone, which we 

 have shown circular, though the shape is generally far from being so regular. If 

 such a system be established over our imaginary area of sun-heated air, then it will 

 be seen that the anticyclonic winds will prevent the formation of local cyclones, and 



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



