358 



SCIENCE PROGRESS 



current of air blowing on a steep cliff face (fig. 7). We get a 

 vigorous ascending current and, at the top of the cliff, an 

 eddy of remarkable properties. Let any one try the experiment 



by standing on a cliff edge which faces a 

 gale of wind ; he will find that he can throw 

 his hat over the cliff without losing it. An 

 empty match-box is cheaper — that often comes 

 — " back to him after describing a vertical circular 

 course. In the laboratory small balls of cotton 

 wool answer admirably. Under the influence of the eddy they 

 are first drawn towards the cliff and then thrown upward by 

 the current. 



Next let us take some important phenomena which depend 

 upon the want of homogeneity of broad air currents. The 



Fig. 7. 



February 19, 1903. 



February 22, 1903. 



February 20, 1903. 



Trajectories 

 February 19 to 22. 



February 21, 1903. 



Trajectories 

 February 18 to 21. 



Fig. 8.— Parallel streams of warm and cold air over the British Isles in. February, 1903. 



north and south sides of a broad air current are sometimes 

 fed from different sources and thus supplied with different air. 

 The thermometer is the best instrument for detecting the want 

 of homogeneity ; we may reasonably assume that a sudden 

 but persistent fall of temperature means drawing the air from 

 a colder supply. 



