II. THE SWALLOW. 69 



qualify my praise considerably, discovering, 

 whe*: I examined the book farther, that the 

 good dof^ior had described the motion of a 

 bird as resembling that of a kite, without ever 

 inquiring what, in a bird, represented that 

 somewhat important part of a kite, the string. 



unquestionably previous investigators. The problem of flight 

 was, to all serious purpose, solved by Borelli in 16S0, and 

 the following passage is very notable as an example of the 

 way in which the endeavour to obscure the light of former 

 ages lOO fatally dims and distorts that by which modern men 

 of science walk, themselves. " Borelli, and all who have 

 written since his time, are unanimous in affirming that the 

 horizontal transference of the body of the bird is due to the 

 perpendicular vibration of the wings, and to the yielding of 

 the posterior or flexible margins of the wings in an upward 

 direction, as the wings descend. 1" (Dr. Pettigrew) "am, 

 however, disposed to attribute it to the fact (ist), that the 

 wings, both when elevated and depressed, leap forwards 

 in cur\'es, those curves uniting to form a continuous waved 

 track; (2nd), to the tendency -which the body of the bird has 

 to swing forwards, in a more or less horizontal direction, 

 when once set in motion; (3rd), to the construction of the 

 wings ; they are elastic helices or screws, which twist and 

 untwist while they vibrate, and tend to bear nfwards and 

 onwards anyweii;ht suspended fro/n them ; (4th), to the action 

 of the air on the under surfaces of the wings ; (5th), to the 

 ever-varying power -with -which the -wings are mged, Ihis 

 being greatest at the beginning of the down-stroke, and least 

 at the end of the up one ; (6lh), to the contraction of the 

 voluntary muscles and clastic ligaments, and to the effect 

 produced by the various inclined surfaces formed by the 

 wings during their oscillations ; (7th), to the -weight of the 

 bird — weight itself, when acting upon wings, becoming 



