120 PROCEEDINGS OF THE AMERICAN ACADEMY 



"Let US describe first how a constant velocity is imparted to the 

 mirror. This mirror, of silvered glass, and 14 millimetres in diameter, 

 is mounted directly upon the axis of a small air-turbine of a well- 

 known model, admirably constructed by Froment. The air is supplied 

 by a high-pressure bellows of Cavaille-Coll, justly distinguished for the 

 manufacture of great organs. As it is important that the pressure 

 should be very constant, the air after leaving the bellows traverses a 

 regulator, recently contrived by Cavaille, in which the pressure does 

 not vary by ^ of a millimetre in a column of water of 30 centimetres. 

 The fluid flowing through the orifices of the turbine represents a motive 

 power of remarkable constancy. On the other hand, the mirror when 

 accelerated soon encounters in the surrounding air a resistance, which, 

 for a given velocity, is also perfectly constant. The moving body 

 placed between these two forces, which tend to equilibrium, cannot fail 

 to receive and to preserve a uniform velocity. Any check whatever, 

 acting upon the flow of the water, allows this velocity to be regulated 

 within very extensive limits. 



" It remains to estimate the number of turns, or rather to impress 

 on the moving body a determined velocity. This problem has been 

 completely resolved in the following manner. Between the microscope 

 and the reflecting glass, a circular disk is placed, the edge of which, cut 

 in fine teeth, encroaches upon the mark and partly intercepts it. The 

 .disc turns uniformly on itself, so that if the image shines steadily, the 

 teeth at its circumference escape detection from the rapidity of the 

 motion. But the image is not pei'manent ; it results from a series of 

 discontinuous appearances, equal in number to the revolutions of the 

 mirror ; and whenever the teeth of the screen succeed one another with 

 the same frequency, there is produced on the eye an illusion easily ex- 

 plained, which makes the teeth appear immovable. Suppose, then, that 

 the disk, with n teeth in its circumference, turns once in a second, and 

 that the turbine starts up. If by regulating the flow of aii', the teeth 

 are made to appear fixed, we are certain that the mirror makes n turns 

 in a second. 



" Froment, who made the turbine, wished to invent and construct a 

 chronometric wheel-work to move the disk. It is a remarkable piece 

 of clock-work, which resolves, in an elegant manner, the problem of 

 uniform motion in the particular case in which there is no work to be 

 done. The success is so complete, that it is my daily experience to 

 launch the mirror with 400 turns a second, and see the two pieces of 



