172 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



accurate determination of the quantities in question, and the 

 methods by which he had been in the highest degree success- 

 ful, Stefan says, " The results so obtained are now in a truly 

 unexpected agreement with the values predicted by Max- 

 well." The conducting power of air is found to be to that 

 of copper as 1 is to 20,000, and to iron as 1 is to 3400. The 

 predicted law that the conductivity of air should be inde- 

 pendent of its density is also experimentally demonstrated 

 in a most irrefragable manner. The conducting power of 

 hydrogen is seven times greater than that of aii', precisely as 

 Maxwell had predicted. But few of the theories of the 

 physical sciences can point to a more brilliant confirmation 

 of predictions than this, and the dynamical theory of gases 

 must now be considered as one of the best established of 

 modern physics. Siizungsber. Acad., Vienna, 1872, 45. 



THE MECHANICAL PRINCirLES OF FLYING. 



1? Aeronaut, a journal devoted to the interests of aerial 

 navigation, contains a valuable contribution to our knowl- 

 edge of the mechanical principles of the flight of birds, in an 

 article by M. A. Penaud. The elaborate experiments of Thi- 

 bault on the resistances opposed by the air to motions of 

 thin plates of metal form the basis of the mathematical studies 

 of Penaud, as well as of those by Louvrie, published in 1868. 

 Thibault's experiments showed that in moving a plane square 

 surface the resistance normal to the surface remains very 

 nearly constant so long as the angle between the normal and 

 the direction of motion (the angle of incidence) is included 

 between 90 and 45; it then diminishes progressively to 20, 

 from which point up to of incidence it is sensibly propor- 

 tional to the sine of the angle. M. Penaud now demonstrates, 

 first, that a bird sailing in the air falls as slowly as possible 

 w^hen he employs for his horizontal movement one fourth of 

 the work of the fall ; second, a bird in sailing with a uniform 

 movement clears a given space with the least possible fall 

 when the work of suspension is sensibly equal to the work 

 of translation : the plane of the wings then bisects the angle 

 formed by the horizon and the direction of movement, and 

 the latter ans^le is itself a minimum. 



From these principles (which apply to birds, and not neces- 

 sarily to insects) Penaud deduces most of the known charac- 



