720 Mr. C. J. P. Cave [April 11, 



ment is given by the rotation of the earth under the moving air. 

 For any pressure condition to be maintained the air must l)e moving 

 with a certain definite velocity, depending on the shape of the isobars 

 and the steepness of the barometric gradient. This rate can be 

 calculated for the conditions obtaining at the time, and the wind so 

 calculated is called the gradient wind. It has been found that there 

 is a fairly good agreement between the wind so calculated and the 

 observed wind at a height of i kilometre or so, but owing to friction 

 the surface wind is usually of a smaller velocity, and directed more 

 towards the low pressure. 



Tn order to show in a clear manner the changes of wind at different 

 levels, I have prepared some models which give a better mental 

 picture of the conditi(jns than a diagram. The atmosphere is sup- 

 posed to be divided up into layers each 1 kilometre thick, and the 

 average wind in each layer is represented by a coloured card ; the 

 length of the card represents tiip velocity of the wind 1 centimetre 

 representing 1 metre per second, 1 metre per second being about 

 2\ miles per hour ; the direction of the card shows the direction of 

 the wind, the arrow flying with the wind. The red cards represent 

 winds that may be supposed to come from an equatorial direction, 

 that is winds from east-south-east through south to west-north-west, 

 the blue cards winds that may be supposed to come from a polar 

 direction. 



For convenience I have divided the wind structures into five 

 types ; they are perhaps rather artificial, as I shall show later, 

 but it is convenient to make some sort of classification, even when 

 further knowledge must change it. In the first three types of wind 

 structure, the wind increases above the surface and equals the gra- 

 dient velocity at a height of i kilometre or so ; above this in 

 the first class the wind remains more or less equal to the gradient 

 velocity, up to a height of 7 or S kilometres ; in the second class 

 the wind in the upper air greatly exceeds the gradient wind, 

 and in the third class it falls off again to a lesser value ; but in all 

 three classes the direction remains much the same as that of the 

 gradient wind. 



The first type may be called the solid current ; it does not seem 

 to be associated with any particular type of isobars, but in a pre- 

 ponderance of cases the wind is easterly, and the remaining cases 

 are nearly all westerly ; it is rare to find the solid current w-ith 

 winds from the north, or from the south. 



In rare cases there is hardly any wind up to the greatest heights 

 reached, and the little wind there is often blows from varying 

 directions in different layers ; this type, which may be looked on as 

 a sub-class of the first type, sometimes occurs in still anti-cyclonic 

 conditions in summer. 



In the second class the gradient wind, after being reached at a 

 height of about h kilometre, is greatly exceeded in the upper air ; in 



