8 Prof. A. Battelli and Mr. L. Magri on 



those used by Feddersen and by Trowbridge and Sabine to 

 enable the spark to be produced at the very moment of the 

 mirror being in convenient position for reflecting its image 

 on the photographic film), we preferred adopting a form of 

 apparatus capable of imparting to the image an extremely 

 great angular velocity and of making r very small. This 

 arrangement, moreover, allowed of fairly luminous images of 

 the spark being obtained, photographic films of moderate 

 size being employed. 



During the first part of our investigations we employed 

 a Froment clockwork, whose last axis could effect about 450 

 turns per second, butnot even this velocity was sufficient in all 

 our researches. Moreover, as is always the case with those 

 instruments where only the friction and resistance of the 

 medium is made use of to regulate the velocity, we found it 

 impossible to maintain the movement constant. 



This defect, inevitable in the case of rotating instruments 

 driven by clockwork, may arouse some doubt as to the 

 accuracy of the results obtained by Miesler. 



For the same reason we rejected our first sets of observa- 

 tions, adopting, for the definite investigations, to produce 

 the rotation of the mirror, a special turbine whose action 

 we found to be perfectly regular. 



Fiff. 1. 



8. This turbine, constructed by the mechanician of our 

 Institute, Mr. Giuseppe Pierucci, as represented by fig. 1, is 

 in its essential parts similar to that of Foucault. 



The steam enters the chamber PP, and leaving it by two 

 openings strikes against the wreath of vanes C, fixed on the 

 spindle A, together with the steel mirror S and the toothed 

 wheel R. 



This latter gears into another perfectly similar wheel R/, 



