Di Bella 



REPLY TO DISCUSSION 



Prof. Alfio Di Bella 



I should like to thank Prof. Poreh for his contribution to the discussion and 

 to write down here some of my considerations. 



To show that results of tests of my devices are in agreement with the prin- 

 ciples of mechanics it is not necessary to draw an analogy with larval insects; 

 it is enough to remember these principles. 



The chapter concerning the motion of any material system in Appell's 

 classical treatise on Rational Mechanics states that: "Le centre de gravite 

 du systeme se meut comme un point materiel, qui aurait pour masse la masse 

 totale du systeme, et auquel seraient appliquees des forces egales et paralleles 

 aux forces exterieures." We can see therefore that any force conditions are 

 acceptable, as long as they are "external." 



The test results of my devices were obtained in the presence of external 

 forces. In fact: 



- In tests on dry ice, friction resistance is the dominating factor; it 

 is very little (practically negligible), but not strictly zero. 



- In tests in water the water pressure acting against the hull is the 

 dominating factor. 



- In tests in air the pressure against the balloon on which the device 

 is placed is the dominating factor. 



For these reasons the working of the devices follows the classical mechanical 

 principles, and the dominating factor in my experiments is not the friction 

 coefficient alone. 



However, it would be interesting to know the minimum values of the ex- 

 ternal forces which are necessary to prevent the device from working. Theo- 

 retically, this could be done by writing the motion equations of the device; and 

 in a practical way, by carrying out tests in the presence of external forces 

 that gradually decrease to zero. 



As far as the power absorbed by the device is concerned, it has not been 

 possible to find out experimentally if the power diminishes with the external 

 forces. The 140-gram device works with the same small battery both on dry 

 ice and on a wood table. We should remember, however, that the device is 

 subject to vibrations and shocks on the supporting plain. 



At present, it is difficult to say how power absorbed by the device is 

 distributed. 



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