328 PHILOSOPHICAL TRANSACTIONS. [anNO 1747. 



tainty. But if the immenseness of the globe be considered, and the vast tracts 

 of land which still remain unknown to us, it is no wonder that we are yet unac- 

 quainted with the retreat of these itinerant birds. On this head Mr. C. agrees 

 in the general opinion of their passing to other countries by the common natural 

 way of flying, with this additional conjecture, that the places to which they re- 

 tire, lie probably in the same latitude in the southern hemisphere, as the places 

 from whence they depart; where the seasons reverting, they may enjoy the like 

 agreeable temperature of air. 



It may be objected, that places of the same latitude in the southern hemisphere 

 may be divided by too wide a tract of sea for them to pass over. But why then 

 may not some other parts of the southern hemisphere serve their turn? This 

 seems more reasonable, than that they should remain on our side of the northern 

 tropic; within a few degrees of which, at the winter solstice, it is so cold, as 

 frequently to produce snow; which, by dispersing such insects as birds that feed 

 on the wing, and particularly the swallow kinds, subsist on, must make them 

 perish inevitably, were they not to change their quarters for those more favour- 

 able climes, where a continuance of warm weather affords their natural and pro- 

 per food. This their sagacity dictates to them, and is the apparent cause of their 

 periodically leaving us at the approach of winter, before flies are so dissipated by 

 cold and winds as to be found no longer in the air, though they may with other 

 insects be met with in holes and hidden recesses, and serve to subsist other birds 

 of passage. 



What Mr. C. infers from hence is, that as swallows cannot continue and sub- 

 sist so long in cold seasons as other birds of passage, they are necessitated to visit 

 us somewhat later, and to depart sooner; for though nightingales, and other birds 

 of passage, are not often seen or observed after they cease singing, yet he has 

 frequently noticed them in their solitary coverts a month after the departure of 

 swallows. From these reasons he concludes, that birds of passage, particularly 

 swallows, are necessitated to pass the tropic of Cancer ; but how far more south, 

 or to what part of the southern hemisphere they go, remains unknown. 



The manner of their journeying to their southern abode may vary, as the 

 different structure of their bodies enables them to support themselves in the air: 

 those birds with short wings, such as the red-start, blackcap, &c. though they 

 are incapable of such long flights, and with so much celerity, yet he thinks they 

 may pass in like manner, but by gradual and slower movements. Swallows and 

 cuckows may probably perform their flight in half the time; yet there seems no 

 necessity for a precipitate passage, because every day's passage affords them in- 

 crease of warmth, and a continuance of food a longer time than is necessary 

 for their passage, were it to the same latitude south as that from whence 

 they go. 



