Page:The Migration of Birds - Thomas A Coward - 1912.pdf/111

Rh 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—Fair Island, the Flannens, the Isle of May, and Heligoland 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 areal "With all our great army of trained observers," 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