ZOOLOGY. 227 



its arrival to its departure is usually only three 

 months. 



The common swallow appears in Sussex about the 

 third week in April ; in Edinburgh, about the second 

 week in May. 



The cuckow does not build any nest, but lays her 

 eggs in the nests of other birds, but not indiscrimi- 

 nately, selecting those nests only which belong to 

 birds that have bills of the same kind with herself, 

 and consequently feed on the same kind of food. 



Canary Birds. 



Dr. Fordyce found, that when Canary birds, at the 

 time of laying their eggs, are deprived of all access 

 to carbonate of lime, (common limestone,) or other 

 calcareous matter, the eggs produced have soft shells. 



Rapid Flight of Birds. 



The rapidity with which the hawk and many other 

 birds occasionally fly, is probably not less than at the 

 rate of . . . . 150 miles in an hour. 



The common crow, 25 ditto. 



A swallow . . 92 ditto, and the swift three 

 times greater.* 



Migratory birds probably about 50 miles per hour. 



The Bones and Muscles of Birds. 



If any quantity of matter is to be fabricated into a 

 rod of a certain length, the rod will be strong in pro- 

 portion to its thickness ; and if the figure remains the 

 same, that thickness can only be increased by making 

 it hollow. Therefore, hollow rods or tubes of the 

 same length and quantity of matter, have more 

 strength than solid ones of less diameter. Now the 

 bones of animals are all more or less hollow; but 



* This appears scarcely possible. 

 Q2 



