308 Froceedinr/s of tlic Eoyal Fhysical Society. . 



the normal conditions of our Western Isles and tlieir mete- 

 orology. But should easterly winds prevail in autumn, or 

 south-easterly, abnormal results are sure to follow, east winds 

 being abnormal at that season. 



The wave of bird-migration in our reports has been likened 

 to d^fan. The normal lines of flight we have seen are E. to 

 W.. or south of E. to north of W. ; but if the wind be northerly, 

 or north of W., the effect is to press down to a great extent 

 the rays of the fan, or cause what, in our reports, has been 

 termed a close fan of migration. Accordingly to the extent 

 of strength and northering of westerly winds will the closeness 

 of the fan be. 



If, on the other hand, easterly winds prevail, following^ 

 overtaking the migrants, according to the extent of its 

 southering and strength will the opening out of the wave or 

 fan of migration be, causing what we call an o'pen fan of 

 migration. If a succession of easterly gales takes place, 

 thousands of migrants are carried away over the Atlantic 

 and drowned in the ocean ; but if a succession of KW. gales 

 occur at the same season, the flights of migrants are beaten 

 down, compressed, and obliged to fly low over their routes. 

 If, again, light westerly winds prevail, migrants travel easiest^ 

 reach furthest, feel least exhausted, and normally extend 

 their range furthest to the westward and north-westward; 

 but if easterly winds prevail, even if light and regular, the 

 disassorting of feathers and the weariness caused by the loss 

 of steering power forces a rapid search for rest and shelter 

 immediately on finding land, and thus an abnormal restric- 

 tion is placed upon their extension of range; lecause, after 

 resting, they pursue a more southerly course over land routes, 

 and do not seek to move further westward. 



Birds migrating voluntarily with the normal, ix., beam- 

 winds, fly loiv and steady. Birds migrating ^voluntarily 

 with the a&normal — wind astern — are borne away higher, 

 nolens volens, losing all power of steering, and are either lost, 

 or, reaching land, sink exhausted to recover strength. 



(d.) The vast altitude at which an American astronomer 

 has ascertained birds to travel throws considerable light upon 

 migration. It has been recorded that birds were actually 



