CONTRIVANCE A NECESSITY. I 73 



more strongly with one wing than with the other* 

 To this theory there are two objections — first, 

 that as a matter of fact birds can turn, and do 

 turn, even to the extent of describing complete 

 circles in the air, without any flapping either of 

 one wing or the other; and secondly, that when 

 birds do flap and turn at the same time, not the 

 slightest difference in time between the two wing- 

 strokes can ever be detected. The beats of a bird's 

 two wings are always exactly synchronous. But 

 the first of these two objections is of itself quite 

 sufficient to disprove the theory. No man can have 

 watched even for a moment the flight of the com- 

 mon Swallow, and especially the flight of the Swift, 

 without seeing it perform complete gyrations in the 

 air without any strokes of either wing. The only 

 change which can ever be detected by the eye is a 



* Referring to a Boat he says : "Si remi dexteri lateris celerius 

 quam sinistri aquam retrorsum impellant — semper prora revolvetur 

 versus sinistrum latus; ergo eodem modo dum avis in medio fluido 

 aeris innatat, volando asquilibrata in centra gravitatis ejus, si sola 

 dextra ala deorsum sed oblique flectatur, aerem subjectum impel- 

 lando versus caudam necessario ad instar navis mox memoratse, 

 permovetur latus ejus dextrum, quiescente aut tardius moto sinistra 

 latere. Ex quo fit, ut avis pars anterior circa centrum gravitatis 

 ejus revoluta, flectatur versum sinistrum latus." — Borettus, De Motu 

 Animalium, Pars Prima. Propositio cxcix. 



