362 SCIENCE PROGRESS 



deeper and more malignant significance. You know the short, 

 violent, spiral gusts that lift the dust before the coming rain : 

 the Harpies get identified first with these and then with more 

 violent whirlwinds and so they are called 'Harpies,' 'the 

 Snatchers ' and are thought of as entirely destructive ; their 

 manner of destroying being twofold — by snatching away and 

 by defiling and polluting. This is a month in which you may 

 really see a small Harpy at her work almost whenever you 

 choose. The first time that there is threatening of rain after 

 two or three days of fine weather, leave your window well 

 open to the street and some books or papers on the table; 

 and if you do not, in a little while, know what the Harpies mean 

 and how they snatch and how they defile, I'll give up my 

 Greek myths." 



The small Harpy to which Ruskin refers is probably the 

 shower-squall or comparatively mild type of line-squall, 

 characterised by its slight rise of pressure, change of wind (with 

 a squall), fall of temperature and shower of rain. Whether 

 or not the idea of pollution may be associated with the notable 

 darkening of the sky which is very characteristic of these showers 

 I must leave to some one sufficiently familiar with Greek myths. 



Thus we may summarise the structure of the atmosphere : 

 At the surface a current running, up to 80 miles an hour 

 in very exceptional cases (with eddies formed by all suitably 

 shaped obstacles in the way of the flow) generally increasing 

 with elevation in proportion to the height above sea level up 

 to the gradient wind ; and above that level, different kinds of 

 structure on different occasions — from the " solid " current, which 

 keeps the same speed and direction at all levels, to one which, at 

 higher levels, within 10,000 ft. shows winds of increased velocity 

 or a calm or a reversal or a cross wind; the vertical structure dis- 

 turbed by local ascending or descending columns of air the 

 positions of which may be indicated by detached clouds, 

 between which there must be a distributed adjustment of level 

 in operation in order to make up for the vertical displacement 

 of the local column : the broader currents made up of hetero- 

 geneous streams running alongside, which sometimes, from their 

 juxtaposition, give rise to the destructive phenomena of the 

 line-squall. 



Texture of Air Currents 



For the texture of air currents we may appeal to the records 

 of pressure tube anemometers which show by the breadth of 



