PAPER BY PROF. HELMHOLTZ 73 



but the volume of the body and of the muscles that do the work in- 

 creases only in the ratio ( i. ) 



Hence it follows that the size of a bird has a limit, unless the muscles 

 can be further developed in such a manner that for the same mass as 

 now they shall perform more work. Now it is precisely among the 

 larger birds, that are capable of the greater performances in flying, 

 that we find those that eat only flesh and fish ; they are animals that 

 consume concentrated food and need no extensive system of diges- 

 tive organs. Among the smaller birds many grain eaters like doves 

 and the smaller singing birds are also good flyers. It therefore ap- 

 pears probable that in the model of the great vulture, nature has al- 

 ' ready reached the limit that can ba attained with the muscles as work- 

 ing organs, and under the most favorable conditions of subsistence, 

 for the magnitude of a creature that shall raise itself by its wings* and 

 remain a long time in the air. 



Under these circumstances it is scarcely to be considered as probable 

 that man even by means of the most ingenious wing-like mechanism 

 that must be moved by his own muscles will ever possess the strength 

 needed to raise his own weight in the air and continue there. 



As concerns the question as to the possibility of driving balloons for- 

 ward relative to the surrounding air, our propositions allow us to com- 

 pare this problem with the other one that is practically executed in 

 many forms, namely, to drive a ship forwards in water by means of 

 oar-like or screw-like organs of motion. In studying this we must not 

 consider movement on the surface, but rather imagine to ourselves a 

 ship driven along under the surface. But such a balloon which pre 

 sents a surface above and below that is congruent with the submerged 

 surface of an ordinary ship scarcely differs in its powers of motion from 

 an ordinary ship. 



If now we let the small letters of the two above given systems of 

 hydro-dynamic equations refer to water and the large letters to the 

 air, then for 0° temperature and 7G0 mm. of the barometer, we have 



-=773 

 r 



According to the determination of O. E. Meyer and Gierke Maxwell, 



4=0.8082 

 the velocity of sound gives for n the value 



w=0.2314 

 Hence the increase of linear dimensions is 



9 =3.4928 

 n 



*[That is, by the work done by its wings; this of course does not cover the case of 

 soaring birds whose muscles do no lifting work but simply keep the wings in the best 

 position for the wind to act on them. — ft A.~\ 



