63 



then will the northern letter be above 20 degrees to the 

 west of the true north*. 



Similar tables of the autumnal diminution of their 

 numbers and latest appearances might be made out. 

 By a comparison of many such tables as these, made 

 from observations in different places, may we possibly 

 trace the swallow, in some degree, through its passage, 

 by noticing its successive appearance in places more 

 and more northward from the aequatorial to the polar 

 regions in spring, and backward again in autumn. Such 

 tables, made out in different places, may be communi- 

 cated to the public in the Philosophical Magazine, and 

 other periodical journalst. 



* The compassnorth is constantly varying. See the excellent 

 tables of the variation of the magnetic needle, by Colonel Beaufoy, 

 in Thomson's " Annals of Philosophy." 



f I shall endeavour, henceforward, to publish an account of the 

 first and latest appearance of migratory birds, through the medium 

 of the Philosophical Magazine, edited by Mr. Alexander Tilloch. 

 And I recommend this Journal as the medium of communications 

 of this sort from distant parts. 



