MIGRATION AND WEATHER 89 



which were passing above the range of normal 

 vision. Mr Eagle Clarke, commenting upon the 

 extraordinary numbers of rare and exceptional 

 visitors which are noticed on many islands Pair 

 Island, the Flannens, the Isle of May, and Heligo- 

 land may be taken as a few examples says that 

 it is their detached position and comparatively 

 small size which makes these islands so useful 

 to the observer. The same variety of birds and 

 greater numbers reach larger islands and tracts of 

 land, but they are unobserved when they are thinly 

 distributed and not massed or confined in a small 

 area. " With all our great army of trained ob- 

 servers/' he declares, " we in Britain see only an 

 infinitesimal number of the migrants which visit 

 our shores ..." and " this is especially the case 

 on the mainland." 



During an anticyclone there is a descending 

 movement of air currents from a centre of high 

 pressure in all directions, and these currents or 

 winds are deflected " clockwise " in the northern 

 hemisphere ; and when cyclonic conditions prevail 

 the air currents are directed inwards towards a 

 low-pressure central area, rotating spirally at the 

 surface of the earth in the direction contrary to the 

 hands of a watch. In the southern hemisphere the 

 directions are reversed. A cyclonic system is 

 usually carried forward by great drift winds like 



